Properly recurse when removing "unsafe" from inlinable code

I forgot that I have to manually recurse in the syntactic rewriter. Do so.
Fixes rdar://147877042

test/Unsafe/module-interface.swift

@@ -23,7 +23,9 @@ public struct SequenceWithUnsafeIterator: Sequence {
   // CHECK-NOT: unsafe
   print( unsafe getIntUnsafely())
 
-  for unsafe _ in SequenceWithUnsafeIterator() { }
+  for unsafe _ in SequenceWithUnsafeIterator() {
+    _ = unsafe getIntUnsafely()
+  }
 }
 
 // CHECK: public protocol P