lang-items: Cleanup parsing and lookups of lang items.

lang-items: Cleanup parsing and lookups of lang items.

gcc/rust/ChangeLog:

	* Make-lang.in: Compile new rust-lang-item.o.
	* util/rust-lang-item.h: Split into header and source.
	* util/rust-lang-item.cc: Cleanup parsing of lang items by using a hashmap
	and returning optional values, cleanup handling of exhaustive lang item
	lookups.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::handle_lang_item_attribute): Use
	new optional API.