Migrate the target attribute parsing code into an extension off of

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