[stdlib] MutableCollection fix to _SmallString

Author: glessard

stdlib/public/core/SmallString.swift

@@ -191,11 +191,16 @@ extension _SmallString: RandomAccessCollection, MutableCollection {
 
   @inlinable  @inline(__always)
   internal subscript(_ bounds: Range<Index>) -> SubSequence {
-    // TODO(String performance): In-vector-register operation
-    return self.withUTF8 { utf8 in
-      let rebased = UnsafeBufferPointer(rebasing: utf8[bounds])
-      return _SmallString(rebased)._unsafelyUnwrappedUnchecked
+    get {
+      // TODO(String performance): In-vector-register operation
+      return self.withUTF8 { utf8 in
+        let rebased = UnsafeBufferPointer(rebasing: utf8[bounds])
+        return _SmallString(rebased)._unsafelyUnwrappedUnchecked
+      }
     }
+    // This setter is required for _SmallString to be a valid MutableCollection.
+    // Since _SmallString is internal and this setter unused, we cheat.
+    @_alwaysEmitIntoClient set { fatalError() }
   }
 }