libclang: audit all APIs that accept a CXTranslationUnit to make sure that
checks for invalid translation unit are in place.  Also, while there, add log
output for this case.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201155 91177308-0d34-0410-b5e6-96231b3b80d8