Allow non-ASCII characters in Rust identifiers

Allow non-ASCII characters in Rust identifiers

Rust 1.53 (quite a while ago now) ungated the support for non-ASCII
identifiers.  This didn't work in gdb.  This is PR rust/20166.

This patch fixes the problem by allowing non-ASCII characters to be
considered as identifier components.  It seemed simplest to just pass
them through -- doing any extra checking didn't seem worthwhile.

The new test also verifies that such characters are allowed in strings
and character literals as well.  The latter also required a bit of
work in the lexer.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20166