* c-common.c, c-decl.c, c-lex.c, c-objc-common.c, c-opts.c,
	c-parse.in, c-pragma.c, c-typeck.c: Use %q, %< and %> for quoting
	in diagnostics.  Use %' for English apostrophes.
	* c-tree.h (ATTRIBUTE_GCC_CDIAG): Define.
	(pedwarn_c90, pedwarn_c99): Use it.
	* c-decl.c (lookup_label): Quote label name in diagnostic.
	* c-parse.in (yyprint): Use ' instead of ` for left quote.
	* c-typeck.c (warn_for_assignment): Likewise.

testsuite:
	* g++.dg/warn/Wswitch-1.C, g++.dg/warn/Wswitch-2.C,
	gcc.dg/Wswitch-2.c, gcc.dg/Wswitch-enum.c, gcc.dg/Wswitch.c,
	gcc.dg/declspec-9.c, gcc.dg/declspec-10.c, gcc.dg/declspec-11.c,
	gcc.dg/m-un-2.c, gcc.dg/noreturn-1.c, gcc.dg/wtr-conversion-1.c:
	Update expected messages.

From-SVN: r87413