TextDiagnosticPrinter.cpp: Show diagnostics as far as possible even with invalid PresomedLoc, instead of just silencing it.

FileManager.cpp: Allow virtual files in nonexistent directories.
FileManager.cpp: Close FileDescriptor for virtual files that correspond to actual files.
FileManager.cpp: Enable virtual files to be created even for files that were flagged as NON_EXISTENT_FILE, e.g. by a prior (unsuccessful) addFile().

ASTReader.cpp: Read a PCH even if the original source files cannot be found.

Add a test for reading a PCH of a file that has been removed and diagnostics referencing that file.

llvm-svn: 124374