Sema: address post-commit review comment

Hoist the IgnoreParens so that we ignore it around attributes as well in order
to future-proof the code.  Addresses Richard's comments for SVN r219974.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220053 91177308-0d34-0410-b5e6-96231b3b80d8