[clang][Interp] Implement bitcasts

[tbaeder]

Summary

I really don't know much about these, so limited tests included.

[tbaeder]

Note that this "implementation" is based on my knowledge about what bitcasts are, so no idea. Is that documented anywhere or should I check ExprConstant.cpp, or is this implementation fine for the time being (IIRC this was needed to make a stdlib header work)?

[shafik]

I think VisitCastExpr is the right place to start looking for what to do.

[tbaeder]

Abandoning this since it's superseded by https://reviews.llvm.org/D144943

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
   case CK_ArrayToPointerDecay:
   case CK_AtomicToNonAtomic:
   case CK_ConstructorConversion:
   case CK_FunctionToPointerDecay:
   case CK_NonAtomicToAtomic:
   case CK_NoOp:
   case CK_UserDefinedConversion:
+  case CK_BitCast:
     return this->visit(SubExpr);
 
   case CK_IntegralToBoolean:
     // == in C
     if (const auto *BOP = dyn_cast<BinaryOperator>(SubExpr);
         BOP && BOP->isEqualityOp())
       return this->visit(SubExpr);
     [[fallthrough]];

clang/test/AST/Interp/literals.cpp

 static_assert(~e == -1, "");
 
 
 constexpr int m = 10;
 constexpr const int *p = &m;
 static_assert(p != nullptr, "");
 static_assert(*p == 10, "");
 
+constexpr const void *cp = (void *)p;
+// FIXME: This should be an error in the new interpreter.
+constexpr const int *pm = (int*)cp; // ref-error {{ must be initialized by a constant expression}} \
+                                    // ref-note {{cast from 'const void *' is not allowed}}
+
 constexpr const int* getIntPointer() {
   return &m;
 }
 static_assert(getIntPointer() == &m, "");
 static_assert(*getIntPointer() == 10, "");
 
 constexpr int gimme(int k) {
   return k;