[Clang][OpenMP] Fix the issue that a functor is not captured properly in a task region

This patch fixes the issue that a functor is not captured properly if
that is used in a task region. It was introduced by https://reviews.llvm.org/D114546
where `CallExpr` is treated specially, but the callee itself is not properly visited.
https://reviews.llvm.org/D115902 already did some fix for one case. This patch
fixes another case.

Fix #57757.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D141873