When clang crashes, it writes a standalone source file and shell script
to reproduce the crash.
The Driver used to set Mode = CPPMode in generateCompilationDiagnostics()
to force preprocessing mode. This has the side effect of making
IsCLMode() return false, which in turn meant Clang::AddClangCLArgs()
didn't get called when creating the standalone source file, which meant
the stand-alone file was preprocessed with the gcc driver's defaults
In particular, exceptions default to on with the gcc driver, but to
off with the cl driver. The .sh script did use the original command
line, so in the reproducer for a clang-cl crash, the standalone source
file could contain exception-using code after preprocessing that the
compiler invocation in the shell script would then complain about.
This patch removes the Mode = CPPMode; line and instead additionally
checks for CCGenDiagnostics in most places that check `CCCIsCPP().
This also matches the strategy Clang::ConstructJob() uses to add
-frewrite-includes for creating the standalone source file for a crash