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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

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"?

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