Change lookup_macro_invocation's return type

Change lookup_macro_invocation's return type

Wrap the function's return type within an optional and remove the out
reference argument.

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.cc (MacroExpander::expand_invoc): Adapt
	the function call to match its new prototype.
	* resolve/rust-early-name-resolver-2.0.cc (Early::insert_once):
	Likewise.
	(Early::visit): Likewise.
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit):
	Likewise.
	* util/rust-hir-map.cc (Mappings::lookup_macro_invocation): Change the
	function's return type.
	* util/rust-hir-map.h: Update the function's prototype.

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