Implement P0892R2, explicit(bool).

Implement P0892R2, explicit(bool).

	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_explicit_bool.

	* call.c (add_template_candidate_real): Return if the declaration is
	explicit and we're only looking for non-converting constructor.
	* cp-tree.h (lang_decl_fn): Add has_dependent_explicit_spec_p bit.
	(DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P): New macro.
	(cp_decl_specifier_seq): Add explicit_specifier field.
	(build_explicit_specifier, store_explicit_specifier): Declare.
	* decl.c (grokdeclarator): Call store_explicit_specifier.
	(build_explicit_specifier): New function.
	* parser.c (cp_parser_function_specifier_opt) <case RID_EXPLICIT>:
	Parse C++20 explicit(bool).
	* pt.c (store_explicit_specifier, lookup_explicit_specifier): New.
	(tsubst_function_decl): Handle explicit(dependent-expr).

	* g++.dg/cpp2a/explicit1.C: New test.
	* g++.dg/cpp2a/explicit10.C: New test.
	* g++.dg/cpp2a/explicit11.C: New test.
	* g++.dg/cpp2a/explicit12.C: New test.
	* g++.dg/cpp2a/explicit13.C: New test.
	* g++.dg/cpp2a/explicit2.C: New test.
	* g++.dg/cpp2a/explicit3.C: New test.
	* g++.dg/cpp2a/explicit4.C: New test.
	* g++.dg/cpp2a/explicit5.C: New test.
	* g++.dg/cpp2a/explicit6.C: New test.
	* g++.dg/cpp2a/explicit7.C: New test.
	* g++.dg/cpp2a/explicit8.C: New test.
	* g++.dg/cpp2a/explicit9.C: New test.

	* testsuite/20_util/any/cons/explicit.cc: Adjust dg-error.
	* testsuite/20_util/pair/cons/explicit_construct.cc: Likewise.
	* testsuite/20_util/tuple/cons/explicit_construct.cc: Likewise.

From-SVN: r265641