[clang] Fix aggregate initialization inside lambda constexpr

Constant evaluator only considered access to `this` pointer to be
possible if `this` poitners was captured. However `this` can also appear if
there was a default member initializer.

Fixes https://github.com/llvm/llvm-project/issues/60936

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D144866