[stdlib] Make String.Index(_:within:) initializers more permissive

In Swift 5.6 and below, (broken) code that acquired indices from a
UTF-16-encoded string bridged from Cocoa and kept using them after a
`makeContiguousUTF8` call (or other mutation) may have appeared to be
working correctly as long as the string was ASCII.

Since https://github.com/apple/swift/pull/41417, the
`String(_:within:)` initializers recognize miscoded indices and reject
them by returning nil. This is technically correct, but it
unfortunately may be a binary compatibility issue, as these used to
return non-nil in previous versions.

Mitigate this issue by accepting UTF-16 indices on a UTF-8 string,
transcoding their offset as needed. (Attempting to use an UTF-8 index
on a UTF-16 string is still rejected — we do not implicitly convert
strings in that direction.)

rdar://89369680

stdlib/public/core/StringUnicodeScalarView.swift

@@ -429,13 +429,13 @@ extension String.UnicodeScalarIndex {
     within unicodeScalars: String.UnicodeScalarView
   ) {
     guard
-      unicodeScalars._guts.hasMatchingEncoding(sourcePosition),
-      sourcePosition._encodedOffset <= unicodeScalars._guts.count,
-      unicodeScalars._guts.isOnUnicodeScalarBoundary(sourcePosition)
+      let i = unicodeScalars._guts.ensureMatchingEncodingNoTrap(sourcePosition),
+      i._encodedOffset <= unicodeScalars._guts.count,
+      unicodeScalars._guts.isOnUnicodeScalarBoundary(i)
     else {
       return nil
     }
-    self = sourcePosition
+    self = i
   }
 
   /// Returns the position in the given string that corresponds exactly to this