Add TargetRegistry::lookupTarget.

 - This is a simplified mechanism which just looks up a target based on the
   target triple, with a few additional flags.

 - Remove getClosestStaticTargetForModule, the moral equivalent is now:
     lookupTarget(Mod->getTargetTriple, true, false, ...);

 - This no longer does the fuzzy matching with target data (based on endianness
   and pointer width) that getClosestStaticTargetForModule was doing, but this
   was deemed unnecessary.

llvm-svn: 77111