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, 2 insertions, 0 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 3aeca23cd..ab22108b9 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -1581,6 +1581,7 @@ namespace Slang
         if(!witnessTable)
         {
             witnessTable = new WitnessTable();
+            witnessTable->baseType = DeclRefType::create(m_astBuilder, interfaceDeclRef);
         }
         context->mapInterfaceToWitnessTable.Add(interfaceDeclRef, witnessTable);
 
@@ -2137,6 +2138,7 @@ namespace Slang
             // let them define a tag value with the name `__Tag`).
             //
             RefPtr<WitnessTable> witnessTable = new WitnessTable();
+            witnessTable->baseType = enumConformanceDecl->base.type;
             enumConformanceDecl->witnessTable = witnessTable;
 
             Name* tagAssociatedTypeName = getSession()->getNameObj("__Tag");