[FileManager] getFile(open=true) after getFile(open=false) should open the file.

Summary:
Old behavior is to just return the cached entry regardless of opened-ness.
That feels buggy (though I guess nobody ever actually needed this).

This came up in the context of clangd+clang-tidy integration: we're
going to getFile(open=false) to replay preprocessor actions obscured by
the preamble, but the compilation may subsequently getFile(open=true)
for non-preamble includes.

Reviewers: ilya-biryukov

Subscribers: ioeric, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54691

llvm-svn: 347205