Fix unconditional dereference of a WeakVH in CGDebugInfo TypeCache

This fails an "isa<> used with null pointer" assert during a clang-cl
self-host on Windows.  This was caused by r202769, and I'm currently
reducing a test case.

Reviewers: dblaikie

Differential Revision: http://llvm-reviews.chandlerc.com/D2944

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202888 91177308-0d34-0410-b5e6-96231b3b80d8