c-common.c, [...]: Use %q, %< and %> for quoting in diagnostics.
* 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.