Be careful when printing line comments with incorrect
position information. Maintain additional state
impliedSemi: when set, a comment containing a newline
would imply a semicolon and thus placement must be
delayed.

Precompute state information pertaining to the next
comment for faster checks (the printer is marginally
faster now despite additional checks for each comment).

No effect on existing src, misc sources.

Fixes #1505.

R=rsc
CC=golang-dev
https://golang.org/cl/5598054