contracts: Lowercase {MAYBE,NEVER}_CONTINUE

The lowercase constants are more consistent with the standard, and it is
unlikely that the uppercase versions would've been accepted.

gcc/cp/ChangeLog:

	* contracts.cc: Rename references to
	contract_violation_continuation_mode constants to be lowercase.

libstdc++-v3/ChangeLog:

	* include/experimental/contract: Lowercase the constants in
	contract_violation_continuation_mode.

gcc/cp/contracts.cc

@@ -41,9 +41,9 @@ along with GCC; see the file COPYING3.  If not see
 	 "v > 0", // comment,
 	 "default", // assertion_level,
 	 "default", // assertion_role,
-	 MAYBE_CONTINUE, // continuation_mode
+	 maybe_continue, // continuation_mode
        });
-       terminate (); // if NEVER_CONTINUE
+       terminate (); // if never_continue
      }
 
    We use an internal type with the same layout as contract_violation rather

libstdc++-v3/include/experimental/contract

@@ -45,7 +45,7 @@ namespace experimental
 {
   // From P1332
   enum class contract_violation_continuation_mode {
-    NEVER_CONTINUE, MAYBE_CONTINUE
+    never_continue, maybe_continue
   };
 
   class contract_violation {