[clang] [clang] Fix crash when declaring invalid lambda member (PR #74110)

[Nico Weber via cfe-commits]

nico wrote:

Also on the official waterfall, eg here: https://lab.llvm.org/buildbot/#/builders/188/builds/42935

Can you take a look, and revert for now if it takes a while to fix?

https://github.com/llvm/llvm-project/pull/74110