Revert "[ABI] [C++20] [Modules] Don't generate vtable if the class is defined in other module unit"

Breaks check-clang on win and mac, see comments on https://reviews.llvm.org/D150023

This reverts commit d8a36b0.

Also revert follow-up "[NFC] skip the test modules-vtable.cppm on windows"
This reverts commit baf0b12.

Author: nico

clang/lib/CodeGen/CGVTables.cpp

@@ -1172,16 +1172,9 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
   if (!keyFunction)
     return false;
 
-  const FunctionDecl *Def;
   // Otherwise, if we don't have a definition of the key function, the
   // vtable must be defined somewhere else.
-  if (!keyFunction->hasBody(Def))
-    return true;
-
-  assert(Def && "The body of the key function is not assigned to Def?");
-  // If the non-inline key function comes from another module unit, the vtable
-  // must be defined there.
-  return Def->isInAnotherModuleUnit() && !Def->isInlineSpecified();
+  return !keyFunction->hasBody();
 }
 
 /// Given that we're currently at the end of the translation unit, and