[libclang] Don't try to translate diagnostics from the precompiled preamble to the

code-completion results, the SourceManager state may be slightly
different when code-completing.

And we don't even care for diagnostics when code-completing, anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170979 91177308-0d34-0410-b5e6-96231b3b80d8