c++: implement __remove_cv, __remove_reference and __remove_cvref

c++: implement __remove_cv, __remove_reference and __remove_cvref

This implements builtins for std::remove_cv, std::remove_reference and
std::remove_cvref using TRAIT_TYPE from the previous patch.

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_reswords): Add __remove_cv,
	__remove_reference and __remove_cvref.
	* c-common.h (enum rid): Add RID_REMOVE_CV, RID_REMOVE_REFERENCE
	and RID_REMOVE_CVREF.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Handle CPTK_REMOVE_CV,
	CPTK_REMOVE_REFERENCE and CPTK_REMOVE_CVREF.
	* cp-objcp-common.cc (names_builtin_p): Likewise.
	* cp-tree.h (enum cp_trait_kind): Add CPTK_REMOVE_CV,
	CPTK_REMOVE_REFERENCE and CPTK_REMOVE_CVREF.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle CPTK_REMOVE_CV,
	CPTK_REMOVE_REFERENCE and CPTK_REMOVE_CVREF.
	* parser.cc (cp_keyword_starts_decl_specifier_p): Return true
	for RID_REMOVE_CV, RID_REMOVE_REFERENCE and RID_REMOVE_CVREF.
	(cp_parser_trait): Handle RID_REMOVE_CV, RID_REMOVE_REFERENCE
	and RID_REMOVE_CVREF.
	(cp_parser_simple_type_specifier): Likewise.
	* semantics.cc (finish_trait_type): Likewise.

libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (unique_ptr<_Tp[], _Dp>): Remove
	__remove_cv and use __remove_cv_t instead.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Test existence of __remove_cv,
	__remove_reference and __remove_cvref.
	* g++.dg/ext/remove_cv.C: New test.
	* g++.dg/ext/remove_reference.C: New test.
	* g++.dg/ext/remove_cvref.C: New test.