[llvm] [LangRef] inline asm: the instructions are treated opaquely (PR #157080)

[Jacob Lifshay via llvm-commits]

programmerjake wrote:

> That would require changing the clang frontend to support a nonstandard C/C++ `asm directive`. We don't want to require the user to write in a nonstandard language dialect.

using just [standard C++11](https://en.cppreference.com/w/cpp/language/asm.html):
```c++
[[amdgpu::sgmask("...")]] asm ("instructions_like_normal");
```

https://github.com/llvm/llvm-project/pull/157080