[rfa] Add SYMBOL_SET_LINKAGE_NAME

[Paul N. Hilfinger]

> Did you end up with basenames or with the equivalent of "short"
> demangled names?

Neither for now.  At the moment, we still do the same old thing of simply 
keeping around the whole linkage name (well, WE don't; that's just standard
GDB) and only demangling when needed.   So I am just using the 
language-specific demangled name to cache the demangled name with a 
sufficiently long lifetime.  This required keeping track of an appropriate
obstack---hence my need for the same type-tag field you used.  

As I said when you first mentioned the scheme that you appear to be working on 
here, it sounds as if we'd be able to dispense with our current 
space-and-time-saving Ada-specific code and adapt your implementation instead.
Hence, my interest.

Paul Hilfinger