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.