CompatbilitySpan: Add a missing @unsafe.

The missing attribute was causing "no unsafe expressions" warnings elsewhere
when building the CompatibilitySpan library.

Author: tshortli

stdlib/toolchain/CompatibilitySpan/FakeStdlib.swift

@@ -74,10 +74,11 @@ internal func _overrideLifetime<
 }
 
 extension Range {
-    @_alwaysEmitIntoClient
-	internal init(_uncheckedBounds bounds: (lower: Bound, upper: Bound)) {
-	    self.init(uncheckedBounds: bounds)
-	}
+  @unsafe
+  @_alwaysEmitIntoClient
+  internal init(_uncheckedBounds bounds: (lower: Bound, upper: Bound)) {
+    self.init(uncheckedBounds: bounds)
+  }
 }
 
 extension Optional {
@@ -90,4 +91,4 @@ extension Optional {
       _internalInvariantFailure("_unsafelyUnwrappedUnchecked of nil optional")
     }
   }
-}
+}