[Frontend/Modules] Show diagnostics on prebuilt module configuration mismatch too

The current version only emits  the below error for a module (attempted to be loaded) from the `prebuilt-module-path`:

```
error: module file blabla.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
```

With this change, if the prebuilt module is used, we allow the proper diagnostic behind the configuration mismatch to be shown.

```
error: POSIX thread support was disabled in PCH file but is currently enabled
error: module file blabla.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
```

(A few lines later an error is emitted anyways, so there is no reason not to complain for configuration mismatches if a config mismatch is found and kills the build.)

Reviewed By: dblaikie

Tags: #clang

Differential Revision: https://reviews.llvm.org/D53334

llvm-svn: 346439