[sema] Check the result of getAsCXXRecordDecl() to fix the build

I'm not 100% sure what should happen here to find the real
CXXRecordDecl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178297 91177308-0d34-0410-b5e6-96231b3b80d8