Teach compiler to allow constexpr in asm block

[pedrolamarao]

constexpr allows the compiler to treat a name as-if a literal.

Currently, LLVM does not allow something like this.

constexpr auto size = sizeof(something);

[[gnu::naked]]
void foo () {
  __asm__ {
    mov eax, size
  }
}

It would be incredibly useful if it did.