[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 one-line fix to `SynthesizedFunctionScope`) that also makes `PushExpressionEvaluationContextForFunction` cleaner by no longer needing the check for lambda methods. Basically it boils down to the "defect" fix + fixing a bug in `SynthesizedFunctionScope` not escalating to immediate functions properly.

I will clean this up into a nice issue+PR with tests and post it tomorrow.

My local branch already passes all tests, including the example by @efriedma-quic and some variants.

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