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

[Logan Chien]

Hi Tim,

AFAIK, the naming convention is something like "ld."+name, e.g.

* ld.bfd for BFD linker
* ld.gold for Gold
* ld.mcld for MCLinker

Thus, it won't be the problem for these linkers.  Although, I am not sure
about lld.

Alternatively, I can slightly extend this option to search both the given
name and "ld."+name.  However, I am not sure whether this is a good idea or
not.

Sincerely,
Logan

http://reviews.llvm.org/D4295