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, 3 insertions, 0 deletions

diff --git a/source/slang/slang-emit.cpp b/source/slang/slang-emit.cpp
index 3e0d6ae26..2212d5d5a 100644
--- a/source/slang/slang-emit.cpp
+++ b/source/slang/slang-emit.cpp
@@ -698,6 +698,9 @@ SlangResult emitEntryPointSourceFromIR(
         //
         // TODO: do we want to emit directly from IR, or translate the
         // IR back into AST for emission?
+#if 0
+        dumpIR(compileRequest, irModule, "PRE-EMIT");
+#endif
         sourceEmitter->emitModule(irModule);
     }