[Preprocessor] Don't avoid entering included files after hitting a fatal error.

Change in r337953 violated the contract for `CXTranslationUnit_KeepGoing`:

> Do not stop processing when fatal errors are encountered.

Use different approach to fix long processing times with multiple inclusion
cycles. Instead of stopping preprocessing for fatal errors, do this after
reaching the max allowed include depth and only for the files that were
processed already. It is likely but not guaranteed those files cause a cycle.

rdar://problem/46108547

Reviewers: erik.pilkington, arphaman

Reviewed By: erik.pilkington

Subscribers: jkorous, dexonsmith, ilya-biryukov, Dmitry.Kozhevnikov

Differential Revision: https://reviews.llvm.org/D55095

llvm-svn: 348641