[Clang] only inherit the parent eval context inside of lambdas (#124426)
As we create defaul constructors lazily, we should not inherit from the
parent evaluation context.
However, we need to make an exception for lambdas (in particular their
conversion operators, which are also implicitly defined).
As a drive-by, we introduce a generic way to query whether a function is
a member of a lambda.
This fixes a regression introduced by baf6bd3.
Fixes #118000