[Clang][NFC] assert IFaceT in SemaObjC::HandleExprPropertyRefExpr (llvm#138026)

Static analysis flagged that we use IFaceT in HandleExprPropertyRefExpr
without checking even though getInterfaceType() can return nullptr. The
comments make it clear the assumption is that we will always have an
interface, so we will document this via an assert.

Fixes: llvm#134954

Author: shafik

clang/lib/Sema/SemaExprObjC.cpp

@@ -1980,6 +1980,7 @@ ExprResult SemaObjC::HandleExprPropertyRefExpr(
     SourceLocation SuperLoc, QualType SuperType, bool Super) {
   ASTContext &Context = getASTContext();
   const ObjCInterfaceType *IFaceT = OPT->getInterfaceType();
+  assert(IFaceT && "Expected an Interface");
   ObjCInterfaceDecl *IFace = IFaceT->getDecl();
 
   if (!MemberName.isIdentifier()) {