Clarify that effective-targets 'exceptions' and 'exceptions_enabled' are orthogonal

In Subversion r268025 (Git commit 3f21b8e3f7)
"Add dg-require-effective-target exceptions", effective-target 'exceptions'
was added, which "says that AMD GCN does not support [exception handling]".

In Subversion r279246 (Git commit a9046e9853)
"MSP430: Add -fno-exceptions multilib", effective-target 'exceptions_enabled'
was added "to check if the testing configuration supports exceptions".  Testing
"if exceptions are unsupported or disabled (e.g. by passing -fno-exceptions)"
works as expected if exception handling is disabled at the front-end level
('-fno-exceptions'; the "exceptions are [...] disabled" case):

    exceptions_enabled2066068.cc: In function ‘void foo()’:
    exceptions_enabled2066068.cc:3:27: error: exception handling disabled, use ‘-fexceptions’ to enable

However, effective-target 'exceptions_enabled' additionally assumes that
"If exceptions aren't supported [by the target], then they're not enabled".
This is not correct: it's not unlikely that, in presence of explicit/implicit
'-fexceptions', exception handling code gets fully optimized away by the
compiler, and therefore effective-target 'exceptions_enabled' test cases may
PASS even for targets that don't support effective-target 'exceptions'; these
two effective-targets are orthogonal concepts.

(For completeness: code with trivial instances of C++ exception handling may
translate into simple '__cxa_allocate_exception', '__cxa_throw' function calls
without requiring any back end-level "exceptions magic", and then trigger
unresolved symbols at link time, if these functions are not available.)

This change only affects GCN, as that one currently is the only target declared
as not supporting effective-target 'exceptions'.

	gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords): Clarify that
	effective-target 'exceptions' and 'exceptions_enabled' are
	orthogonal.
	gcc/testsuite/
	* lib/gcc-dg.exp (gcc-dg-prune): Clarify effective-target
	'exceptions_enabled'.
	* lib/target-supports.exp
	(check_effective_target_exceptions_enabled): Don't consider
	effective-target 'exceptions'.
	libstdc++-v3/
	* testsuite/lib/prune.exp (libstdc++-dg-prune): Clarify
	effective-target 'exceptions_enabled'.

gcc/doc/sourcebuild.texi

@@ -2996,11 +2996,12 @@ Target uses @code{__cxa_atexit}.
 Target has packed layout of structure members by default.
 
 @item exceptions
-Target supports exceptions.
+Target supports exception handling.
+Note that this is orthogonal to effective-target @code{exceptions_enabled}.
 
 @item exceptions_enabled
-Target supports exceptions and they are enabled in the current
-testing configuration.
+Testing configuration has exception handling enabled.
+Note that this is orthogonal to effective-target @code{exceptions}.
 
 @item fgraphite
 Target supports Graphite optimizations.

gcc/testsuite/lib/gcc-dg.exp

@@ -434,8 +434,7 @@ proc gcc-dg-prune { system text } {
 	return "::unsupported::large return values"
     }
 
-    # If exceptions are disabled, mark tests expecting exceptions to be enabled
-    # as unsupported.
+    # If exception handling is disabled, expectant tests are UNSUPPORTED.
     if { ![check_effective_target_exceptions_enabled] } {
 	if [regexp "(^|\n)\[^\n\]*: error: exception handling disabled" $text] {
 	    return "::unsupported::exception handling disabled"

gcc/testsuite/lib/target-supports.exp

@@ -12559,7 +12559,8 @@ proc check_effective_target_fenv_exceptions_long_double {} {
     } [add_options_for_ieee "-std=gnu99"]]
 }
 
-# Return 1 if -fexceptions is supported.
+# Check whether the target supports exception handling.
+# Note that this is orthogonal to effective-target 'exceptions_enabled'.
 
 proc check_effective_target_exceptions {} {
     if { [istarget amdgcn*-*-*] } {
@@ -12568,23 +12569,20 @@ proc check_effective_target_exceptions {} {
     return 1
 }
 
-# Used to check if the testing configuration supports exceptions.
-# Returns 0 if exceptions are unsupported or disabled (e.g. by passing
-# -fno-exceptions). Returns 1 if exceptions are enabled.
+# Check whether the testing configuration has exception handling enabled.
+# Returns 0 if exception handling is disabled (e.g. by passing
+# -fno-exceptions). Returns 1 if exception handling is enabled.
+# Note that this is orthogonal to effective-target 'exceptions'.
+
 proc check_effective_target_exceptions_enabled {} {
     return [check_cached_effective_target exceptions_enabled {
-	if { [check_effective_target_exceptions] } {
-	    return [check_no_compiler_messages exceptions_enabled assembly {
-		// C++
-		void foo (void)
-		{
-		    throw 1;
-		}
-	    }]
-	} else {
-	    # If exceptions aren't supported, then they're not enabled.
-	    return 0
-	}
+	return [check_no_compiler_messages exceptions_enabled assembly {
+	    // C++
+	    void foo (void)
+	    {
+		throw 1;
+	    }
+	}]
     }]
 }
 

libstdc++-v3/testsuite/lib/prune.exp

@@ -89,8 +89,7 @@ proc libstdc++-dg-prune { system text } {
     # the single uncapitalized "in function" line.
     regsub -all "(^|\n)\[^\n\]*: in function\[^\n\]*" $text "" text
 
-    # If exceptions are disabled, mark tests expecting exceptions to be enabled
-    # as unsupported.
+    # If exception handling is disabled, expectant tests are UNSUPPORTED.
     if { ![check_effective_target_exceptions_enabled] } {
 	if [regexp "(^|\n)\[^\n\]*: error: exception handling disabled" $text] {
 	    return "::unsupported::exception handling disabled"