This patch introduces additional infrastructure necessary to accommodate DiagnosticOptions.
TheyDiagnosticOptions are unique in that they are parsed in -cc1by the same function in cc1 AND in the Clang driver. The call to the parsing function from the driver occurs early on in the compilation process, but also in the **driver** (`driver.cpp: CreateAndPopulateDiagOpts` & `clang::ToolInvocation::run`)where no proper DiagnosticEngine exists, because the diagnostic options (passed through command line) are not known yet.
To preserve the current behavior, we need to be able to selectively parse:
* all options (for -cc1),
* only diagnostic options (for driver).
The idea is that the TableGen backend checks whether the `Keypath` of option record starts with `DiagnosticOpts.`. If that's so, the backend emits the definition inside a macro prefixed with `DIAG_`, making it possible to distinguish between regular and diagnostic option definitionsis patch achieves that in the following way:
* new MacroPrefix field is added to the Option TableGen class,
* new IsDiag TableGen mixin sets MacroPrefix to "DIAG_",
* TableGen backend serializes option records into a macro with the prefix,
* CompilerInvocation parse/generate methods define the [DIAG_]OPTION_WITH_MARSHALLING macros to handle diagnostic options separately.
Depends on D93700, D93701 & D937012.