[stdlib] StringProtocol._toUTF16Indices: Speed up conversion of short ranges

Instead of calling `_toUTF16Index` twice, call it once and then use
`index(_:offsetBy:)` to potentially avoid another breadcrumbs lookup.

(cherry picked from commit 2423b8b)

Author: lorentey

stdlib/public/core/StringBridge.swift

@@ -807,7 +807,7 @@ extension StringProtocol {
   public // SPI(Foundation)
   func _toUTF16Indices(_ range: Range<Int>) -> Range<Index> {
     let lowerbound = _toUTF16Index(range.lowerBound)
-    let upperbound = _toUTF16Index(range.lowerBound + range.count)
+    let upperbound = self.utf16.index(lowerbound, offsetBy: range.count)
     return Range(uncheckedBounds: (lower: lowerbound, upper: upperbound))
   }
 }