This is an add-on to the RFC at http://lists.llvm.org/pipermail/cfe-dev/2020-May/065421.html
This introduces optional additional information in Option records that can
specify a mapping between an option and a field of a class (used here for
CompilerInvocation).
Also it adds a GenerateCC1CommandLine method to Compiler invocation that uses
the TableGen definitions to generate command line argument strings.
Is this really a should not, or is it must not?