Make [[clang::nomerge]] work for trap intrinsics such as __debugbreak and __builtin_trap

[rnk]

Consider:
https://gcc.godbolt.org/z/esvPqdfn9

#define NOMERGE [[clang::nomerge]]
bool condition();
void mergeChecks() {
    if (condition())
      NOMERGE __builtin_trap();
      if (condition())
      NOMERGE __builtin_trap();
      if (condition())
      NOMERGE __builtin_trap();
}

LLVM folds all these traps together:

_Z11mergeChecksv: # @_Z11mergeChecksv
  push rax
  call _Z9conditionv
  test al, al
  jne .LBB0_4
  call _Z9conditionv
  test al, al
  jne .LBB0_4
  call _Z9conditionv
  test al, al
  jne .LBB0_4
  pop rax
  ret
.LBB0_4: # %if.then
  ud2

Ideally, the nomerge attribute should prevent this. Currently the attribute only really works with call expressions.

[davidben]

We currently use an inline asm block in Chromium, basically to work around not being able to say this in CHECKs. If [[clang::nomerge]] __builtin_trap() worked, I suspect we could switch to that!

[rnk]

@ZequanWu started work on this in https://reviews.llvm.org/D146164 , but I think it stalled out over the concern of extending support to all the other builtins which generate calls (__builtin_printf, etc).