[NFC][Migrator] Make Diff.h private (#28637)

Gardening: `Migrator` is the reason Diff.h is here. Reduce its
visibility.

Author: dduan

include/swift/Basic/Diff.h renamed to lib/Migrator/Diff.h

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "swift/Basic/Diff.h"
+#include "Diff.h"
 #include "swift/Frontend/Frontend.h"
 #include "swift/Migrator/ASTMigratorPass.h"
 #include "swift/Migrator/EditorAdapter.h"

lib/Migrator/Migrator.cpp

@@ -227,7 +227,7 @@ There are two main other pieces of the Migrator's implementation, *diffing* and
 
 For diffing, we pulled in an STL port of Google's *diff-match-patch* library to perform the final diff of the start and end `MigrationState`'s text. This is a fairly standard implementation of the Myers Difference Algorithm (see *An O(ND) Difference Algorithm and Its Variations* by Eugene W. Myers).
 
-> See include/swift/Basic/Diff.h
+> See Diff.h
 
 ### Editing