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/typinf.cpp

@@ -439,8 +439,7 @@ void buildTypeInfo(TypeInfoDeclaration *decl) {
     // immutable on the D side, and e.g. synchronized() can be used on the
     // implicit monitor.
     const bool isConstant = false;
-    // TODO: no dllimport when compiling druntime itself
-    const bool useDLLImport = isBuiltin && global.params.dllimport;
+    bool useDLLImport = isBuiltin && global.params.dllimport != DLLImport::none;
     gvar = declareGlobal(decl->loc, gIR->module, type, irMangle, isConstant,
                          false, useDLLImport);
   }