[libclang] Fix CXTranslationUnit_KeepGoing

Since
  commit 56f548bbbb7e4387a69708f70724d00e9e076153
  [modules] Round-trip -Werror flag through explicit module build.
the behavior of CXTranslationUnit_KeepGoing changed:
Unresolved #includes are fatal errors again. As a consequence, some
templates are not instantiated and lead to confusing errors.

Revert to the old behavior: With CXTranslationUnit_KeepGoing fatal
errors are mapped to errors.

Patch by Nikolai Kosjar.

Differential Revision: https://reviews.llvm.org/D58501

llvm-svn: 355586