Mark non-foreign entry points of @objc dynamic methods in generic classes dynamically_replaceable

```
class Generic<T> {
  @objc dynamic func method() {}
}

extension Generic {
  @_dynamicReplacement(for:method())
  func replacement() {}
}
```

The standard mechanism of using Objective-C categories for dynamically
replacing @objc methods in generic classes does not work.

Instead we mark the native entry point as replaceable.

Because this affects all @objc methods in generic classes (whether there
is a replacement or not) by making the native entry point
`[dynamically_replaceable]` (regardless of optimization mode) we guard this by
the -enable-implicit-dynamic flag because we are late in the release cycle.

* Replace isNativeDynamic and isObjcDynamic by calls to shouldUse*Dispatch and
  shouldUse*Replacement
  This disambiguates between which dispatch method we should use at call
  sites and how these methods should  implement dynamic function
  replacement.

* Don't emit the method entry for @_dynamicReplacement(for:) of generic class
  methods
  There is not way to call this entry point since we can't generate an
  objective-c category for generic classes.

rdar://63679357

lib/Serialization/SerializedModuleLoader.cpp

@@ -690,6 +690,8 @@ FileUnit *SerializedModuleLoaderBase::loadAST(
       M.setTestingEnabled();
     if (extendedInfo.arePrivateImportsEnabled())
       M.setPrivateImportsEnabled();
+    if (extendedInfo.isImplicitDynamicEnabled())
+      M.setImplicitDynamicEnabled();
 
     auto diagLocOrInvalid = diagLoc.getValueOr(SourceLoc());
     loadInfo.status =