c++: streamline built-in trait addition process

Adding a new built-in trait currently involves manual boilerplate
consisting of defining an rid enumerator for the identifier as well as a
corresponding cp_trait_kind enumerator and handling them in various switch
statements, the exact set of which depends on whether the proposed trait
yields (and thus is recognized as) a type or an expression.

To streamline the process, this patch adds a central cp-trait.def file
that tabulates the essential details about each built-in trait (whether
it yields a type or an expression, its code, its spelling and its arity)
and uses this file to automate away the manual boilerplate.  It also
migrates all the existing C++-specific built-in traits to use this
approach.

After this change, adding a new built-in trait just entails declaring
it in cp-trait.def and defining its behavior in finish_trait_expr/type
(and handling it in diagnose_trait_expr, if it's an expression-yielding
trait).

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_reswords): Use cp/cp-trait.def to handle
	C++ traits.
	* c-common.h (enum rid): Likewise.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Likewise.
	* cp-objcp-common.cc (names_builtin_p): Likewise.
	* cp-tree.h (enum cp_trait_kind): Likewise.
	* cxx-pretty-print.cc (pp_cxx_trait): Likewise.
	* parser.cc (cp_keyword_starts_decl_specifier_p): Likewise.
	(cp_parser_primary_expression): Likewise.
	(cp_parser_trait): Likewise.
	(cp_parser_simple_type_specifier): Likewise.
	* cp-trait.def: New file.

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