Stop trying to reuse imported Clang types. Required for opaque pointe…

…r migration.

src/tcompiler.cpp

@@ -586,25 +586,13 @@ class Types {
         return true;
     }
     StructType *CreateStruct(Obj *typ) {
-        // check to see if it was initialized externally first
-        if (typ->boolean("llvm_definingfunction")) {
-            const char *name = typ->string("llvm_definingfunction");
-            size_t target_id = typ->number("llvm_definingtarget");
-            TerraTarget *TT = T->targets[target_id];
-            assert(TT);
-            // Important: only use the externally defined type if is was
-            // defined in the current target
-            if (TT == CU->TT) {
-                Function *df = TT->external->getFunction(name);
-                assert(df);
-                int argpos = typ->number("llvm_argumentposition");
-                StructType *st = cast<StructType>(df->getFunctionType()
-                                                          ->getParamType(argpos)
-                                                          ->getPointerElementType());
-                assert(st);
-                return st;
-            }
-        }
+        // Note: historically, Terra tried to reuse the types generated by Clang when
+        // importing C headers. This is why we maintain an `llvm_definingfunction` and
+        // `llvm_definingtarget`. As of the opaque pointer migration (circa LLVM 15-17),
+        // this approach no longer works. But it turns out that we don't need it: we can
+        // just define the structs below, as we've always been doing (and found was
+        // required for external targets).
+
         std::string name = typ->asstring("name");
         bool isreserved = beginsWith(name, "struct.") || beginsWith(name, "union.");
         name = (isreserved) ? std::string("$") + name : name;