Here's a mildly insane test case reduced from mozilla.

$ cat t.cc
#define NS_CONSTRUCTOR_FASTCALL __attribute__ ((regparm (3), stdcall))

struct nsCOMPtr_base {
  NS_CONSTRUCTOR_FASTCALL ~nsCOMPtr_base() {}
};

struct nsCOMPtr : private nsCOMPtr_base {};

int main() {
  nsCOMPtr ptr;
}

$ clang -target i386-linux-gnu -O1 -Xclang -disable-llvm-optzns -flto -S -o - t.cc
define i32 @main() #0 {
entry:
  %ptr = alloca %struct.nsCOMPtr, align 1
  call void bitcast (void (%struct.nsCOMPtr_base*)* @_ZN13nsCOMPtr_baseD2Ev to void (%struct.nsCOMPtr*)*)(%struct.nsCOMPtr* %ptr)
  ret i32 0
}

define linkonce_odr x86_stdcallcc void @_ZN13nsCOMPtr_baseD2Ev(%struct.nsCOMPtr_base* inreg %this)



Here we create a call to @_ZN13nsCOMPtr_baseD2Ev without adding the x86_stdcallcc attribute to the call. The optimizer will turn this into a call to @llvm.trap.