c++: Improve dump_decl for standard concepts

c++: Improve dump_decl for standard concepts

This patch improves the pretty printing of standard concept definitions in error
messages.  In particular, standard concepts are now printed qualified whenever
appropriate, and the "concept" specifier is printed only when the
TFF_DECL_SPECIFIERS flag is specified.

In the below test, the first error message changes from
  9:15: error: ‘b’ was not declared in this scope; did you mean ‘concept b’?
to
  9:15: error: ‘b’ was not declared in this scope; did you mean ‘a::b’?

gcc/cp/ChangeLog:

	* error.c (dump_decl) [CONCEPT_DECL]: Use dump_simple_decl.
	(dump_simple_decl): Handle standard concept definitions as well as
	variable concept definitions.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts6.C: New test.