Basic/Diagnostics: Add three explicit bits to DiagnosticMappingInfo to track...

Basic/Diagnostics: Add three explicit bits to DiagnosticMappingInfo to track "no-warning-as-error", "no-error-as-fatal", and "show-in-system-header", and update DiagnosticsEngine::setDiagnosticGroup{WarningAsError,ErrorAsFatal} and GetDefaultDiagMappingInfo to set them appropriately.
 - No actual functionality change for now, we still also use the diag::Mapping::{MAP_WARNING_NO_ERROR,MAP_ERROR_NO_FATAL,MAP_WARNING_SHOW_IN_SYSTEM_HEADER} for a little while longer.

llvm-svn: 140768