gccrs: session: Add tokenstream dump option

gccrs: session: Add tokenstream dump option

Add an option to the command line interface to dump the ast back to a
tokenstream.

gcc/rust/ChangeLog:

	* rust-session-manager.cc (Session::enable_dump): Enable new
	dump.
	(Session::compile_crate): CLI argument parsing.
	(Session::dump_tokenstream): Dump the tokenstream as a string in
	the specified file.
	* rust-session-manager.h (struct CompileOptions): Add
	tokenstream dump option.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>