[ASTReader] Only mark module out of date if not already compiled

If a module contains errors (ie. it was built with
-fallow-pcm-with-compiler-errors and had errors) and was from the module
cache, it is marked as out of date - see
a2c1054c.

When a module is imported multiple times in the one compile, this caused
it to be recompiled each time - removing the existing buffer from the
module cache and replacing it. This results in various errors further
down the line.

Instead, only mark the module as out of date if it isn't already
finalized in the module cache.

Reviewed By: akyrtzi

Differential Revision: https://reviews.llvm.org/D100619