Regression for vastart and -fclangir-call-conv-lowering

[ChuanqiXu9]

After #944, the following example may not be accept:

// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM-X86

#include <stdarg.h>
void f1(int n, ...) {
  va_list valist;
  va_start(valist, n);
  va_end(valist);
}

[bcardosolopes]

Unfortunately the error paths are not as great, so I overall disabled it for now: 6cf9c5d