PATCH: minor C++ front end compile speed improvement

[Mark Mitchell]

Matt Austern wrote:
> I'm seeing a bit under 0.5% improvement in QT builds from this.  (Again: 
> close to the limits of accuracy of my measurements, so take that number 
> with a grain of salt.)  We avoid a test and a dereference in 
> namespace_binding when we're looking things up in the global namespace, 
> since we know the global namespace can't be an alias.
> 
> OK to commit to mainline?
> 
>             --Matt

Zack pointed me at this.  Yes, this is OK.  I'm amazed it helps that 
much, but profiling is always amazing.  Add a comment, please, 
explaining why it is safe to do this.  (Not why it's a performance win; 
just why it makes sense.)

THanks,

-- 
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304