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, 5 insertions, 0 deletions

diff --git a/source/slang/slang-emit-cpp.cpp b/source/slang/slang-emit-cpp.cpp
index bc82475f4..d7636e5a8 100644
--- a/source/slang/slang-emit-cpp.cpp
+++ b/source/slang/slang-emit-cpp.cpp
@@ -1559,6 +1559,11 @@ void CPPSourceEmitter::emitParamTypeImpl(IRType* type, String const& name)
     emitType(type, name);
 }
 
+void CPPSourceEmitter::emitWitnessTable(IRWitnessTable* witnessTable)
+{
+    SLANG_UNUSED(witnessTable);
+}
+
 bool CPPSourceEmitter::tryEmitGlobalParamImpl(IRGlobalParam* varDecl, IRType* varType)
 {
     SLANG_UNUSED(varDecl);