CrashTracer: clang at clang: llvm::BitstreamWriter::ExitBlock

Add a guard for re-entering an SDiagsWriter's HandleDiagnostics
method after we've started finalizing. This is a generic catch
all for unexpected fatal errors so we don't recursive crash inside
the generic llvm error handler.

We also add logic to handle the actual error case in
llvm::~raw_fd_ostream caused by failing to clear errors before
it is destroyed.

<rdar://problem/63335596>