gccrs: These are wrappers ported from reusing gccgo

gccrs: These are wrappers ported from reusing gccgo

The wrappers over linemap and location will eventually disappear here but
served as a useful starting point for us. We have wrappers over the
diagnostics system which we might be able to get rid of as well.

	gcc/rust/
	* rust-diagnostics.cc: New.
	* rust-diagnostics.h: New.
	* rust-gcc-diagnostics.cc: New.
	* rust-linemap.cc: New.
	* rust-linemap.h: New.
	* rust-location.h: New.
	* rust-system.h: New.