Revert "[clang] Remove redundant member initialization (NFC)"

This reverts commit 80e2c587.

The original patch causes a lot of warnings on gcc like:

  llvm-project/clang/include/clang/Basic/Diagnostic.h:1329:3: warning:
  base class ‘class clang::StreamingDiagnostic’ should be explicitly
  initialized in the copy constructor [-Wextra]