[indic] Treat U+0A72..0A73 like regular consonants

Unicode 6.x IndicSyllableCategory categorizes them as
placeholders, but they can subjoin.
[indic/myanmar] Refactor a few macros
[myanmar] Use OT_NBSP instead of OT_DOTTEDCIRCLE for OT_GB

No functional change.
[indic/myanmar] Rename OT_NBSP to OT_PLACEHOLDER
[indic] Treat U+00D7 MULTIPLICATION SIGN as placeholder
[indic] Treat U+002D and U+2010..2014 as placeholders
[indic] Clean up a bit

No functional change intended.