c++: tweak colorization of incompatible declspecs

c++: tweak colorization of incompatible declspecs

Introduce a helper function for complaining about "signed unsigned"
and "short long".  Add colorization there so that e.g. the 'signed'
and 'unsigned' are given consistent contrasting colors in both the
message and the quoted source.

gcc/cp/ChangeLog:
	* decl.cc: Add #include "diagnostic-highlight-colors.h"
	and #include "pretty-print-markup.h".
	(complain_about_incompatible_declspecs): New.
	(grokdeclarator): Use it when complaining about both 'signed' and
	'unsigned', and both 'long' and 'short'.

gcc/ChangeLog:
	* diagnostic-highlight-colors.h: Tweak comment.
	* pretty-print-markup.h (class pp_element_quoted_string): New,
	based on pretty-print.cc's selftest::test_element, adding an
	optional highlight color.
	* pretty-print.cc (class test_element): Drop.
	(selftest::test_pp_format): Use pp_element_quoted_string.
	(selftest::test_urlification): Likewise.

gcc/testsuite/ChangeLog:
	* g++.dg/diagnostic/long-short-colorization.C: New test.

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