gccrs: rust: Add -frust-compile-until option

gccrs: rust: Add -frust-compile-until option

This option helps ensure that we do not introduce regressions on various
parts of the compilation pipeline. For example, a testcase (or testsuite
from the `testing` project) might pass attribute checking, expansion and
lowering, but fail during typechecking. Should a change suddenly make
that testcase fail expansion, we would not be able to notice it. By
generating tests that run up until expansion, typechecking, compilation
and so forth we ensure that no regressions are added accidentally to
already failing tests/testsuites.

gcc/rust/ChangeLog:

	* lang.opt: Add new ``-frust-compile-until` option.
	* rust-session-manager.cc (Session::compile_crate): Add stops around
	various compilation steps in the pipeline.
	* rust-session-manager.h (struct CompileOptions): Add `CompileStep` enum
	and field.

gcc/testsuite/ChangeLog:

	* rust/compile/frust-compile-until.rs: New test.