Fix qualified name lookup for Rust

Fix qualified name lookup for Rust

In https://github.com/rust-lang/rust/pull/46457, "m4b" pointed out
that the Rust support in gdb doesn't properly handle the lookup of
qualified names.

In particular, as shown in the test case in this patch, something like
"::NAME" should be found in the global scope, but is not.

This turns out to happen because rust_lookup_symbol_nonlocal does not
search the global scope unless the name in question is unqualified.
However, lookup_symbol_aux does not search the global scope, and
appears to search the static scope only as a fallback (I wonder if
this is needed?).

This patch fixes the problem by changing rust_lookup_symbol_nonlocal
to search the static and global blocks in more cases.

Regression tested against various versions of the rust compiler on
Fedora 26 x86-64.  (Note that there are unrelated failures with newer
versions of rustc; I will be addressing those separately.)

2018-01-19  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_lookup_symbol_nonlocal): Look up qualified
	symbols in the static and global blocks.

2018-01-19  Tom Tromey  <tom@tromey.com>

	* gdb.rust/modules.rs (TWENTY_THREE): New global.
	* gdb.rust/modules.exp: Add ::-qualified lookup test.