[clang] [Clang][Sema] Fix lambdas mistakenly inheriting InImmediateFunctionContext (PR #232458)

[Ferdinand Bachmann via cfe-commits]

Ferdi265 wrote:

I found a way to make this work (needed a single-line fix to `SynthesizedFunctionScope`), and it also makes the `Parent.isConstantEvaluated() || Parent.isImmediateFunctionContext()` check unnecessary, so I think it's cleaner. I will clean this up, make some nice test cases and create an issue+PR for it.

https://github.com/llvm/llvm-project/pull/232458