clang/Modules: Delay err_module_file_conflict if a diagnostic is in flight

clang/Modules: Delay err_module_file_conflict if a diagnostic is in flight

As part of an audit of whether all errors are being reported from the
ASTReader, delay err_module_file_conflict if a diagnostic is already in
flight when it is hit.  This required plumbing an extra argument through
the delayed diagnostic mechanics in DiagnosticsEngine.