Generate IRType for interfaces, and reference them as `operand[0]` in IRWitnessTable values (#1387)

* Generate IRType for interfaces, and  use them as the type of IRWitnessTable values.

This results the following IR for the included test case:
```
[export("_S3tu010IInterface7Computep1pii")]
let  %1 : _     = key
[export("_ST3tu010IInterface")]
[nameHint("IInterface")]
interface %IInterface   : _(%1);

[export("_S3tu04Impl7Computep1pii")]
[nameHint("Impl.Compute")]
func %Implx5FCompute    : Func(Int, Int)
{
block %2(
                [nameHint("inVal")]
                param %inVal    : Int):
        let  %3 : Int   = mul(%inVal, %inVal)
        return_val(%3)
}
[export("_SW3tu04Impl3tu010IInterface")]
witness_table %4        : %IInterface
{
        witness_table_entry(%1,%Implx5FCompute)

}
```

* Fixes per code review comments.

Moved interface type reference in IRWitnessTable from their type to operand[0].

* Fix typo in comment.

1 files changed, 12 insertions, 2 deletions

diff --git a/source/slang/slang-emit-c-like.cpp b/source/slang/slang-emit-c-like.cpp
index 9d23cec43..911928e2a 100644
--- a/source/slang/slang-emit-c-like.cpp
+++ b/source/slang/slang-emit-c-like.cpp
@@ -217,8 +217,8 @@ void CLikeSourceEmitter::emitSimpleType(IRType* type)
         outNumThreads[i] = decor ? Int(getIntVal(decor->getOperand(i))) : 1;
     }
     return decor;
-}
-
+}

+

 void CLikeSourceEmitter::_emitArrayType(IRArrayType* arrayType, EDeclarator* declarator)
 {
     EDeclarator arrayDeclarator;
@@ -265,6 +265,12 @@ void CLikeSourceEmitter::_emitType(IRType* type, EDeclarator* declarator)
 
 }
 
+void CLikeSourceEmitter::emitWitnessTable(IRWitnessTable* witnessTable)
+{
+    SLANG_UNUSED(witnessTable);
+    SLANG_DIAGNOSE_UNEXPECTED(getSink(), SourceLoc(), "Unimplemented emit: IROpWitnessTable.");
+}
+
 void CLikeSourceEmitter::emitTypeImpl(IRType* type, const StringSliceLoc* nameAndLoc)
 {
     if (nameAndLoc)
@@ -3516,6 +3522,10 @@ void CLikeSourceEmitter::emitGlobalInst(IRInst* inst)
         emitStruct(cast<IRStructType>(inst));
         break;
 
+    case kIROp_WitnessTable:
+        emitWitnessTable(cast<IRWitnessTable>(inst));
+        break;
+
     default:
         // We have an "ordinary" instruction at the global
         // scope, and we should therefore emit it using the