Small simplification: Reduce the use of cast<>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208320 91177308-0d34-0410-b5e6-96231b3b80d8
Use more specific type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208321 91177308-0d34-0410-b5e6-96231b3b80d8