core: Rename DECL_IS_BUILTIN -> DECL_IS_UNDECLARED_BUILTIN

In cleaning up C++'s handling of hidden decls, I renamed its
DECL_BUILTIN_P, which checks for loc == BUILTINS_LOCATION to
DECL_UNDECLARED_BUILTIN_P, because the location gets updated, if user
source declares the builtin, and the predicate no longer holds.  The
original name was confusing me.  (The builtin may still retain builtin
properties in the redeclaration, and other predicates can still detect
that.)

I discovered that tree.h had its own variant 'DECL_IS_BUILTIN', which
behaves in (almost) the same manner.  And therefore has the same
mutating behaviour.

This patch deletes the C++ one, and renames tree.h's to
DECL_IS_UNDECLARED_BUILTIN, to emphasize its non-constantness.  I
guess _IS_ wins over _P

	gcc/
	* tree.h (DECL_IS_BUILTIN): Rename to ...
	(DECL_IS_UNDECLARED_BUILTIN): ... here.  No need to use SOURCE_LOCUS.
	* calls.c (maybe_warn_alloc_args_overflow): Adjust for rename.
	* cfgexpand.c (pass_expand::execute): Likewise.
	* dwarf2out.c (base_type_die, is_naming_typedef_decl): Likewise.
	* godump.c (go_decl, go_type_decl): Likewise.
	* print-tree.c (print_decl_identifier): Likewise.
	* tree-pretty-print.c (dump_generic_node): Likewise.
	* tree-ssa-ccp.c (pass_post_ipa_warn::execute): Likewise.
	* xcoffout.c (xcoff_assign_fundamental_type_number): Likewise.
	gcc/c-family/
	* c-ada-spec.c (collect_ada_nodes): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	(collect_ada_node): Likewise.
	(dump_forward_type): Likewise.
	* c-common.c (set_underlying_type): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	(user_facing_original_type, c_common_finalize_early_debug): Likewise.
	gcc/c/
	* c-decl.c (diagnose_mismatched_decls): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	(warn_if_shadowing, implicitly_declare, names_builtin_p)
	(collect_source_refs): Likewise.
	* c-typeck.c (inform_declaration, inform_for_arg)
	(convert_for_assignment): Likewise.
	gcc/cp/
	* cp-tree.h (DECL_UNDECLARED_BUILTIN_P): Delete.
	* cp-objcp-common.c (names_bultin_p): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	* decl.c (decls_match): Likewise.  Replace
	DECL_UNDECLARED_BUILTIN_P with DECL_IS_UNDECLARED_BUILTIN.
	(duplicate_decls): Likewise.
	* decl2.c (collect_source_refs): Likewise.
	* name-lookup.c (anticipated_builtin_p, print_binding_level)
	(do_nonmember_using_decl): Likewise.
	* pt.c (builtin_pack_fn_p): Likewise.
	* typeck.c (error_args_num): Likewise.
	gcc/lto/
	* lto-symtab.c (lto_symtab_merge_decls_1): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	gcc/go/
	* go-gcc.cc (Gcc_backend::call_expression): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	libcc1/
	* libcc1plugin.cc (address_rewriter): Rename
	DECL_IS_BUILTIN->DECL_IS_UNDECLARED_BUILTIN.
	* libcp1plugin.cc (supplement_binding): Likewise.