gccrs: refactor default infer vars to be its own function

gccrs: refactor default infer vars to be its own function

This is just a simple refactor to pull all the logic outside of the
closure which makes it more readable.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check.h: new function
	* typecheck/rust-typecheck-context.cc (TypeCheckContext::compute_inference_variables):
	call the new helper
	(TypeCheckContext::compute_infer_var): refactored code

Signed-off-by: Philip Herron <herron.philip@googlemail.com>