[clang] [clang] Add frontend flag to enable support for broken external resugarers (PR #102510)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> I think using "broken" in the name of the diagnostic and the internal option is a bit problematic -- that's too much of a value judgement, IMO. After all, the previous functionality worked well enough for those folks.
> 
> 
> 
> Would it make more sense to name it something along the lines of "retain substitution ast nodes"?

Thanks. My intention was mostly to discourage users from trying it, without at least looking sideways.

Though I am fine with the name change.

Thanks for fixing the commit up.

https://github.com/llvm/llvm-project/pull/102510