[clang-scan-deps] Add basic support for modules.

This fixes two issues that prevent simple uses of modules from working.

* We would previously minimize _every_ file opened by clang, even module maps
  and module pcm files. Now we only minimize files with known extensions. It
  would be better if we knew which files clang intended to open as a source
  file, but this works for now.

* We previously cached every lookup, even failed lookups. This is a problem
  because clang stats the module cache directory before building a module and
  creating that directory. If we cache that failure then the subsequent pcm
  load doesn't see the module cache and fails.

Overall this still leaves us building minmized modules on disk during scanning.
This will need to be improved eventually for performance, but this is correct,
and works for now.

Differential Revision: https://reviews.llvm.org/D68835

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