gccrs: Compiler proper interface kicks off the pipeline

gccrs: Compiler proper interface kicks off the pipeline

This is a wrapper to get out of C land in the rust-lang.cc and into our
class hierarchy for the rust front-end. We expect that the front-end only
support one source file input as the expansion pass will attempt to resolve
that relative pass and parse accordingly.

The main missing piece here is that we are using saw_errors() to return
early which is unnecessary but as our error handling has been improving
over time we will start to take advantage of error node in our type system
as well as error_mark_node from GCC. The caveat being that our lints/checks
expect no errors and will throw an assertion.

	gcc/rust/
	* rust-lang.cc: New.
	* rust-session-manager.cc: New.
	* rust-session-manager.h: New.