Make bitfields API stable

src/Index.bf

@@ -396,8 +396,8 @@ extension Clang
    * Store PCH in memory. If zero, PCH are stored in temporary files.
    */
   [Bitfield(.Public , .Bits(1), "StorePreamblesInMemory")]
-  [Bitfield(.Private, .Bits(13), "__anon_bitfield_549205FB")]
-  private c_uint __bitfield_EB2E1114;
+  [Bitfield(.Private, .Bits(13), "__anon_bitfield_0")]
+  private c_uint __bitfield_1;
 
   /**
    * The path to a directory, in which to store temporary PCH files. If null or