Driver: rename parameter to reduce confusion

The parameter to ParsePICOpts passed the effective triple and then used
that in a few places and used the actual triple in others.  This was
slightly confusing.  Rename the parameter to make it more obvious.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290303 91177308-0d34-0410-b5e6-96231b3b80d8