ASTReader: Bypass overridden files when reading PCHs

If contents of a file that is part of a PCM are overridden when reading
it, but weren't overridden when the PCM was being built, the ASTReader
will emit an error.  Now it creates a separate FileEntry for recovery,
bypassing the overridden content instead of discarding it.  The
pre-existing testcase clang/test/PCH/remap-file-from-pch.cpp confirms
that the new recovery method works correctly.

This resolves a long-standing FIXME to avoid hypothetically invalidating
another precompiled module that's already using the overridden contents.

This also removes ContentCache-related API that would be unsafe to use
across `CompilerInstance`s in an implicit modules build.  This helps to
unblock us sinking it from SourceManager into FileManager in the future,
which would allow us to delete `InMemoryModuleCache`.

https://reviews.llvm.org/D66710

llvm-svn: 370546