[Myanmar] Prevent reordering between Asat and Dot below

Implemented as a hack for now.  Myanmar failures down from 23 to 15.

MYANMAR: 1123868 out of 1123883 tests passed. 15 failed (0.00133466%)

The remaining 15 cases are all where the syllable is wrong according to
the OpenType spec.  We insert dottedcircle.  Uniscribe fails to do that,
but it also fails to reorder the prebase-reordering medial-Ra.  So it
gets it wrong.

src/hb-unicode-private.hh

@@ -107,6 +107,9 @@ HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS_SIMPLE
   unsigned int
   modified_combining_class (hb_codepoint_t unicode)
   {
+    /* XXX This hack belongs to the Myanmar shaper. */
+    if (unicode == 0x1037) unicode = 0x103A;
+
     return _hb_modified_combining_class[combining_class (unicode)];
   }