Specialize interfaces in DebugFunction (#8617)

E.g. in
[generic-extension-2.slang](https://github.com/shader-slang/slang/blob/master/tests/language-feature/extensions/generic-extension-2.slang),
incorrect DebugFunctions are generated for `getFirstOuter`:

```
let  %33        : Void  = DebugFunction("getFirstOuter", 18 : UInt, 3 : UInt, %26, Func(Int, 0 : Int))
```

This happens because specialization passes are leaving a `%IFoo` in the
function type, instead of replacing with a concrete type:
```
let  %34        : Void  = DebugFunction("getFirstOuter", 18 : UInt, 3 : UInt, %26, Func(Int, %IFoo))
```
and later, `cleanUpInterfaceTypes()` just replaces all interfaces with
the literal zero. So now we have a parameter type which isn't actually a
type at all, but an IntLit instead.

I'm not sure if the approach I picked is good, though. Some other
options that crossed my mind were:
* Make `fixUpFuncType` also update related DebugFunctions
- But is there a reason why DebugFunctions separately carry a function
type in the first place?
* Make `cleanUpInterfaceTypes` less aggressive or at least replace types
with a type instead of a value
    - But this will still make the debug info incorrect :(

1 files changed, 8 insertions, 0 deletions

diff --git a/source/slang/slang-ir-lower-generic-type.cpp b/source/slang/slang-ir-lower-generic-type.cpp
index 9579b0a2b..c4dcd92a9 100644
--- a/source/slang/slang-ir-lower-generic-type.cpp
+++ b/source/slang/slang-ir-lower-generic-type.cpp
@@ -50,6 +50,14 @@ struct GenericTypeLoweringContext
                 structField->setOperand(1, loweredFieldType);
             }
             break;
+        case kIROp_DebugFunction:
+            {
+                auto oldFuncType = as<IRDebugFunction>(inst)->getDebugType();
+                auto newFuncType = sharedContext->lowerType(builder, oldFuncType);
+                if (newFuncType != oldFuncType)
+                    inst = builder->replaceOperand(inst->getOperandUse(4), newFuncType);
+            }
+            break;
         }
         return inst;
     }