Migrate the target attribute parsing code to returning an instance

Migrate the target attribute parsing code to returning an instance
every time it's called rather than attempting to cache the result.
It's unlikely to be called frequently and the overhead of using
it in the first place is already factored out.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246706 91177308-0d34-0410-b5e6-96231b3b80d8