Expose all files included in PCH/PCM to FileManager.

Else, FileManager will fopen(), Preprocessor will notice that there is a defined
code guard and not enter the source file, thus leaving the file handle unclosed.
Now FileEntry will know that a file comes from a PCh/PCM and not open it when
asking for its stat.

(cherry picked from commit 100b2f3f103467bace9ba2b62cde62f70888db47)

Conflicts:
	interpreter/llvm/src/tools/clang/lib/Serialization/ASTReader.cpp