[ASTReader] Always rebuild a cached module that has errors

A module in the cache with an error should just be a cache miss. If
allowing errors (with -fallow-pcm-with-compiler-errors), a rebuild is
needed so that the appropriate diagnostics are output and in case search
paths have changed. If not allowing errors, the module was built
*allowing* errors and thus should be rebuilt regardless.

Reviewed By: akyrtzi

Differential Revision: https://reviews.llvm.org/D95989