Implement Rust raw identifiers

Implement Rust raw identifiers

This patch implements Rust raw identifiers in the lexer in gdb.  There
was an earlier patch to do this, but the contributor didn't reply to
my email asking whether he had sorted out his copyright assignment.

This is relatively straightforward, but a small test suite addition
was needd to ensure that the new test is skipped on older versions of
rustc -- ones that predate the introduction of raw identifiers.

gdb/ChangeLog
2021-06-11  Tom Tromey  <tom@tromey.com>

	PR rust/23427
	* rust-parse.c (rust_parser::lex_identifier): Handle raw
	identifiers.
	(rust_lex_tests): Add raw identifier tests.

gdb/testsuite/ChangeLog
2021-06-11  Tom Tromey  <tom@tromey.com>

	PR rust/23427
	* lib/rust-support.exp (rust_compiler_version): New caching proc.
	* gdb.rust/rawids.exp: New file.
	* gdb.rust/rawids.rs: New file.