[clang][Interp] Don't create global variables more than once

[tbaeder]

Summary

just because we're being told to evaluate it twice. This sometimes
happens when a variable is evaluated again during codegen.

[aaron.ballman]

I keep trying to find some way to test this change... and instead, I've filed several new crashing issues in the issue tracker. The changes look correct to me despite the lack of a test, but if anyone else can dream up some way to tell a ctor is being called twice instead of once, that would be great. If not, LGTM otherwise.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::visitVarDecl(const VarDecl *VD) {
   // We don't know what to do with these, so just return false.
   if (VD->getType().isNull())
     return false;
 
   const Expr *Init = VD->getInit();
   std::optional<PrimType> VarT = classify(VD->getType());
 
   if (shouldBeGloballyIndexed(VD)) {
-    std::optional<unsigned> GlobalIndex = P.getOrCreateGlobal(VD, Init);
+    // We've already seen and initialized this global.
+    if (P.getGlobal(VD))
+      return true;
+
+    std::optional<unsigned> GlobalIndex = P.createGlobal(VD, Init);
 
     if (!GlobalIndex)
       return this->bail(VD);
 
     assert(Init);
     {
       DeclScope<Emitter> LocalScope(this, VD);