classfile reader: allow JDK 9+ constant types in constant pool

forward-port of scala/scala#10675 and scala/scala#8595
references scala/bug#12396 and scala/bug#11635
fixes scala#19527 ("bad constant pool tag 17")
also fixes unreported potential "bad constant pool tag 19" and
"bad constant pool tag 20" errors

[Cherry-picked 26852de][modified]

Author: tgodzik