[Preprocessor] Stop entering included files after hitting a fatal error.

Fixes a problem when we have multiple inclusion cycles and try to
enumerate all possible ways to reach the max inclusion depth.

rdar://problem/38871876

Reviewers: bruno, rsmith, jkorous, aaron.ballman

Reviewed By: bruno, jkorous, aaron.ballman

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D48786

llvm-svn: 337953