Revert "Migrate the target attribute parsing code into an extension off of"

This is failing in release mode. Revert while I figure out what's happening.

This reverts commit r246596.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246598 91177308-0d34-0410-b5e6-96231b3b80d8