Assert pointer cannot be null; NFC

Klocwork static code analysis exposed this concern:
Pointer 'SubExpr' returned from call to getSubExpr() function which may
return NULL from 'cast_or_null<Expr>(Operand)', which will be
dereferenced in the statement following it

Add an assert on SubExpr to make it clear this pointer cannot be null.

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -4348,6 +4348,7 @@ llvm::GlobalVariable *MicrosoftCXXABI::getThrowInfo(QualType T) {
 
 void MicrosoftCXXABI::emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) {
   const Expr *SubExpr = E->getSubExpr();
+  assert(SubExpr && "SubExpr cannot be null");
   QualType ThrowType = SubExpr->getType();
   // The exception object lives on the stack and it's address is passed to the
   // runtime function.