[clang][Interp] Don't assume throw stmts have a subexpr

[tbaeder]

Summary

Totally missed this when I implemented throw statements :(

[cor3ntin]

This one was easy!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::VisitPredefinedExpr(const PredefinedExpr *E) {
   if (DiscardResult)
     return true;
 
   return this->visit(E->getFunctionName());
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitCXXThrowExpr(const CXXThrowExpr *E) {
-  if (!this->discard(E->getSubExpr()))
+  if (E->getSubExpr() && !this->discard(E->getSubExpr()))
     return false;
 
   return this->emitInvalid(E);
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitCXXReinterpretCastExpr(
     const CXXReinterpretCastExpr *E) {

clang/test/AST/Interp/invalid.cpp

static_assert(ConditionalThrow(true) == 0, ""); // expected-error {{not an integral constant expression}} \
                                                   // ref-note {{in call to 'ConditionalThrow(true)'}}
 
   constexpr int Throw() { // expected-error {{never produces a constant expression}} \
                           // ref-error {{never produces a constant expression}}
     throw 5; // expected-note {{subexpression not valid in a constant expression}} \
              // ref-note {{subexpression not valid in a constant expression}}
     return 0;
   }
+
+  constexpr int NoSubExpr() { // ref-error {{never produces a constant expression}} \
+                              // expected-error {{never produces a constant expression}}
+    throw; // ref-note 2{{subexpression not valid}} \
+           // expected-note 2{{subexpression not valid}}
+    return 0;
+  }
+  static_assert(NoSubExpr() == 0, ""); // ref-error {{not an integral constant expression}} \
+                                       // ref-note {{in call to}} \
+                                       // expected-error {{not an integral constant expression}} \
+                                       // expected-note {{in call to}}
 }
 
 namespace Asm {
   constexpr int ConditionalAsm(bool t) {
     if (t)
       asm(""); // expected-note {{subexpression not valid in a constant expression}} \
                // ref-note {{subexpression not valid in a constant expression}}