c-family: Support #pragma region/endregion [PR85487]

These pragmas are used by some editors to mark regions of code for
grouping and folding. GCC should silently ignore them, rather than
giving -Wunknown-pragmas warnings.

	PR c/85487

gcc/ChangeLog:

	* doc/cpp/pragmas.rst (Pragmas): Document region pragmas.

gcc/c-family/ChangeLog:

	* c-pragma.cc (handle_pragma_ignore): New function.
	(init_pragma): Register region and endregion pragmas.

gcc/testsuite/ChangeLog:

	* c-c++-common/pragma-region.c: New test.

gcc/c-family/c-pragma.cc

@@ -1403,6 +1403,12 @@ handle_pragma_message (cpp_reader *)
 	    TREE_STRING_POINTER (message));
 }
 
+/* Ignore a no-op pragma that GCC recognizes, but which has no effect.  */
+static void
+handle_pragma_ignore (cpp_reader *)
+{
+}
+
 /* Mark whether the current location is valid for a STDC pragma.  */
 
 static bool valid_location_for_stdc_pragma;
@@ -1870,6 +1876,9 @@ init_pragma (void)
   c_register_pragma ("GCC", "pop_options", handle_pragma_pop_options);
   c_register_pragma ("GCC", "reset_options", handle_pragma_reset_options);
 
+  c_register_pragma (0, "region", handle_pragma_ignore);
+  c_register_pragma (0, "endregion", handle_pragma_ignore);
+
   c_register_pragma ("STDC", "FLOAT_CONST_DECIMAL64",
 		     handle_pragma_float_const_decimal64);
 

gcc/doc/cpp/pragmas.rst

@@ -119,3 +119,6 @@ GCC plugins may provide their own pragmas.
   file will never be read again, no matter what.  It is a less-portable
   alternative to using :samp:`#ifndef` to guard the contents of header files
   against multiple inclusions.
+
+``#pragma region {tokens}...``, ``#pragma endregion {tokens}...``
+  These pragmas are accepted, but have no effect.

gcc/testsuite/c-c++-common/pragma-region.c

@@ -0,0 +1,11 @@
+/* { dg-options "-Wunknown-pragmas" } */
+/* { dg-final { scan-assembler "code_within_region" } } */
+/* { dg-final { scan-assembler "code_within_named_region" } } */
+
+#pragma region
+void code_within_region() { }
+#pragma endregion
+
+#pragma region ignored name
+void code_within_named_region() { }
+#pragma endregion // ignored comment