Special case "&str" in Rust parser

"&str" is an important type in Rust -- it's the type of string
literals.  However, the compiler puts it in the DWARF in a funny way.
The slice itself is present and named "&str".  However, the Rust
parser doesn't look for types with names like this, but instead tries
to construct them from components.  In this case it tries to make a
pointer-to-"str" -- but "str" isn't always available, and in any case
that wouldn't yield the best result.

This patch adds a special case for &str.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=22251


Reviewed-By: Andrew Burgess <aburgess@redhat.com>