[CIR][CodeGen] Pop null fixups during exception handling cleanup

[tommymcm]

Minimized test case:

#include <string>
#include <vector>
struct a;
int A() {
  std::vector<a> A;
  return 0;
  std::string a;
}

Failed assertion: https://github.com/llvm/clangir/blob/aeac352c9de907fcbb7adde22bbe4a7cfa3105be/clang/lib/CIR/CodeGen/CIRGenCleanup.cpp#L892

[tommymcm]

I am currently minimizing this test case some more to get rid of standard library usage.