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, 3 insertions, 1 deletions

diff --git a/source/slang/slang-ir-specialize.cpp b/source/slang/slang-ir-specialize.cpp
index bc7bcab12..0f8672531 100644
--- a/source/slang/slang-ir-specialize.cpp
+++ b/source/slang/slang-ir-specialize.cpp
@@ -1163,7 +1163,8 @@ struct SpecializationContext
                     // top-down through the program, so that we want to process
                     // the children of an instruction in their original order.
                     //
-                    for (auto child = inst->getLastChild(); child; child = child->getPrevInst())
+                    for (auto child = inst->getLastDecorationOrChild(); child;
+                         child = child->getPrevInst())
                     {
                         // Also note that `addToWorkList` has been written
                         // to avoid adding any instruction that is a descendent
@@ -2055,6 +2056,7 @@ struct SpecializationContext
             }
         }
 
+        fixUpDebugFuncType(newFunc);
         return newFunc;
     }