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.

driver/main.cpp

@@ -1090,10 +1090,10 @@ int cppmain() {
         v == opts::SymbolVisibility::public_ ||
         // default with -shared
         (v == opts::SymbolVisibility::default_ && global.params.dll);
-    global.params.dllimport =
-        v == opts::SymbolVisibility::public_ ||
-        // enforced when linking against shared default libs
-        linkAgainstSharedDefaultLibs();
+    global.params.dllimport = !linkAgainstSharedDefaultLibs() ? DLLImport::none
+                              : v == opts::SymbolVisibility::public_
+                                  ? DLLImport::all
+                                  : DLLImport::defaultLibsOnly;
   }
 
   // allocate the target abi