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

[Yitzhak Mandelbaum via cfe-commits]

ymand wrote:

https://github.com/llvm/llvm-project/pull/103219 has been merged as https://github.com/llvm/llvm-project/commit/661dda9df13c65ce021407bb726b558c7a414731.

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