[myanmar] Relax pwo-tone group a bit

This is broken sequence according to OpenType spec, Uniscribe,
and current HarfBuzz implementation.  But Roozbeh says this
is a valid sequence, so allow it.  There are multiple
"(DB As?)?" constructs in the grammar, but Roozbeh thinks only
this one needs changing.

Test case: 1014,1063,103A

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=71949
1 file changed