<rdar://problem/12068650>

More fixes to how we handle paths that are used to create a target.

This modification centralizes the location where and how what the user specifies gets resolved. Prior to this fix, the TargetList::CreateTarget variants took a FileSpec object which meant everyone had the opportunity to resolve the path their own way. Now both CreateTarget variants take a "const char *use_exe_path" which allows the TargetList::CreateTarget to centralize where the resolving happens and "do the right thing".

llvm-svn: 166186