[clang] Reset track of immediate function context when entering new function

Due to not resetting that, clang still thinks that it is in immediate
function context even if it already entered non-consteval function.
This caused consteval functions reaching codegen in some cases.

Fixes https://github.com/llvm/llvm-project/issues/61142

Reviewed By: cor3ntin, aaron.ballman

Differential Revision: https://reviews.llvm.org/D147531