Branch created for inter-compilation-unit references

[Andrew Cagney]

>>Merging the branch may have to wait until after GDB 6.1.
> 
> 
> This process is looking like gcc, which is probably an improvement.
> Develop on the branch; verify no regressions; then merge.

Careful, GCC is currently faceing an SSA mega-merge.  A strategy, 
reminiscent of the HP merge, is not one we want to encourage here. 
Haveing said that short lived branches for experimentation are a good idea.

Andrew