gccrs: add drops at the end of function bodies

Add function-scope Drop support for the implicit return case.
Add two test cases. This does not yet support explicit return.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (HIRCompileBase::compile_function_body):
	Add drop calls at the end of function bodies.

gcc/testsuite/ChangeLog:

	* rust/execute/drop-function-scope-unit.rs: New test.
	* rust/execute/drop-function-scope.rs: New test.

Signed-off-by: lishin <lishin1008@gmail.com>