gccrs: Infer crate name after file opening

gccrs: Infer crate name after file opening

Fixes #3129.

gcc/rust/ChangeLog:

	* rust-session-manager.cc (Session::handle_crate_name): Remove
	crate name inference
	(Session::compile_crate): Add crate name inference and error if
	inferred name is empty. Remove CompileOptions::get_instance ()
	that returned a local copy of the options. Rename
	crate_name_changed to crate_name_found to match semantics.
	(rust_crate_name_validation_test): Test inferring ".rs" name
	* rust-session-manager.h: Modify handle_crate_name definition to
	include filename.