[llvm] [DirectX][NFC] Model precise overload type specification of DXIL Ops (PR #83917)

[Justin Bogner via llvm-commits]

================
@@ -257,7 +257,7 @@ static FunctionCallee getOrCreateDXILOpFunction(dxil::OpCode DXILOp,
   // FIXME: find the issue and report error in clang instead of check it in
   // backend.
   if ((Prop->OverloadTys & (uint16_t)Kind) == 0) {
-    llvm_unreachable("invalid overload");
+    report_fatal_error("Invalid Overload Type", false);
----------------
bogner wrote:

Generally best to include a comment for boolean flag args:
```c++
    report_fatal_error("Invalid Overload Type", /*gen_crash_diag=*/false);
```

https://github.com/llvm/llvm-project/pull/83917