[LLVM][Support] add nonNull function helper (#188718)
We often see a pattern like:
```
T *ptr = doSomething()
assert(ptr && "doSomething() shouldn't return nullptr");
```
We also have functions like `cantFail`, but those are working with
Expected types.
This commits adds a `nonNull` function, which can be used inline. In
practice, one could use:
```
T *ptr = cast<T>(functionReturningT());
```
But it conveys the meaning that `functionReturningT` might return a
subtype/supertype that we actually cast.
Function behaves like the other error kinds: calls llvm_unreachable,
which may or may not trap depending on the build options. Calling this
function with `nullptr` won't build, but it makes no sense to do so.
---------
Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>
