Windows: Make implicit dllimport more selective

* Newly require `-link-defaultlib-shared` for implicit dllimport.
  E.g., this enables to compile druntime DLL with `-fvisibility=public`
  for pure exports and no (local) imports (such as builtin TypeInfos).
* `-link-defaultlib-shared` alone now only implicitly imports symbols
  from druntime/Phobos.
  This simplifies building complex DLLs linked against a bunch of
  static libs (dub only supports static lib dependencies!); the static
  libs don't need to be compiled with `-fvisibility=public` anymore
  (if the DLL itself isn't either), `-link-defaultlib-shared` is
  sufficient.
  This is mainly useful for existing DLLs with explicit exports, to make
  them link against *shared* druntime/Phobos and so end up with a single
  druntime/Phobos for the whole process.

gen/llvmhelpers.cpp

@@ -1670,6 +1670,22 @@ std::string llvmTypeToString(llvm::Type *type) {
   return result;
 }
 
+bool isDefaultLibSymbol(Dsymbol *sym) {
+  auto mod = sym->getModule();
+  if (!mod)
+    return false;
+
+  auto md = mod->md;
+  if (!md)
+    return false;
+
+  if (md->packages.length == 0)
+    return md->id == Id::object;
+
+  auto p = md->packages.ptr[0];
+  return p == Id::core || p == Id::std || p == Id::etc || p == Id::ldc;
+}
+
 llvm::GlobalVariable *declareGlobal(const Loc &loc, llvm::Module &module,
                                     llvm::Type *type,
                                     llvm::StringRef mangledName,