expand: Add stringify macro

expand: Add stringify macro

Add the stringify macro expansion as well as some tests.

gcc/rust/ChangeLog:

	* ast/rust-macro.cc (builtin_macro_from_string): Add identifier
	identification.
	* ast/rust-macro.h (enum class): Add Stringify builtin macro
	type.
	* expand/rust-macro-builtins.cc (make_macro_path_str): Add path
	for builtin stringify macro.
	(MacroBuiltin::stringify_handler): Add handler for builtin
	stringify macro.
	* expand/rust-macro-builtins.h: Add stringify handler's
	prototype.
	* util/rust-hir-map.cc (Mappings::insert_macro_def): Add
	stringify handler to builtin hir map.

gcc/testsuite/ChangeLog:

	* rust/compile/stringify.rs: Add a basic test with some text.
	* rust/execute/torture/builtin_macro_stringify.rs: Verify the
	text is left as is without any other macro expansion.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>