Fix Bootstrap: UTF8EncoddingError

The pass that annotated control-flow positions unreachable after an
infinite loop was migrated to pure Swift in PR 79186
(https://github.com/swiftlang/swift/pull/79186). As a result, the
C++-only bootstrap compiler is unable to determine that the
code-location is unreachable. Placing a fatalError after the infinite
while loop.

Fixes: rdar://149631113

stdlib/public/core/UTF8EncodingError.swift

@@ -256,6 +256,7 @@ extension UTF8 {
           errors.append(adjustedErr)
         }
       }
+      fatalError()
     }
   }
 }