Migrate the target attribute parsing code into an extension off of

the main attribute and cache the results so we don't have to parse
a single attribute more than once.

This reapplies r246596 with a fix for an uninitialized class member,
and a couple of cleanups and formatting changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246610 91177308-0d34-0410-b5e6-96231b3b80d8