[PATCH] Implement the -fuse-ld= option.

[Tim Northover]

> It's a bit binutils-centric isn't it? Can't we make it accept a
> basename instead, so that any new or otherwise odd (ld.gold-2.25? lld?
> mclinker?) linker can be used instead without modifying Clang.

*Sigh*. Apologies. I've just noticed this comes from GCC land, so
we've probably got to follow for compatibility. After all, why would
anyone want to use anything except the system-default ld.gold or
ld.bfd, they're perfect!

Tim.