c: more precise locations for some -Wpragmas diagnostics

c: more precise locations for some -Wpragmas diagnostics

gcc/c-family/ChangeLog:
	* c-pragma.c (GCC_BAD_AT): New macro.
	(GCC_BAD2_AT): New macro.
	(handle_pragma_pack): Use the location of the pertinent token when
	issuing diagnostics about invalid constants/actions, and trailing
	junk.
	(handle_pragma_target): Likewise for non-string "GCC option".
	(handle_pragma_message): Likewise for trailing junk.

gcc/testsuite/ChangeLog:
	* gcc.dg/bad-pragma-locations.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>