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

[Dmitri Gribenko via cfe-commits]

https://github.com/gribozavr approved this pull request.

Thank you! This change fixes and unblocks our nullability and lifetime tooling. Please merge at your convenience.

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