[MLIR] UnknownLoc on Inlinable Calls in LLVMIR Translation

During MLIR translation to LLVMIR if an inlineable call has an UnkownLoc we get this error message:

```
inlinable function call in a function with debug info must have a !dbg location
  call void @callee()
```

There is code that checks for this case and strips debug information to avoid this situation. I'm expanding this code to handle the case where an debug location points at a UnknownLoc. For example, a NamedLoc whose child location is an UnknownLoc.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D121633