PR43651: add warning for duplicate qualifier

PR43651: add warning for duplicate qualifier

gcc/c/

	PR c/43651
	* c-decl.c (declspecs_add_qual): Warn when -Wduplicate-decl-specifier
	is enabled.
	* c-errors.c (pedwarn_c90): Return true if warned.
	* c-tree.h (pedwarn_c90): Change return type to bool.
	(enum c_declspec_word): Add new enumerator cdw_atomic.

gcc/

	PR c/43651
	* doc/invoke.texi (Wduplicate-decl-specifier): Document new option.

gcc/testsuite/

	PR c/43651
	* gcc.dg/Wduplicate-decl-specifier-c11.c: New test.
	* gcc.dg/Wduplicate-decl-specifier.c: Likewise.

gcc/c-family/

	PR c/43651
	* c.opt (Wduplicate-decl-specifier): New option.

From-SVN: r236142