The parser code was not reachable ever since some of the lexer cleanups.
We could recognize '~' in the lexer, complain, and return a '^' instead,
but it's been a few years since Go was new and this may have been a use-
ful error. The lexer complains with "illegal character U+007E '~'" which
is good enough.

For #13244.

Change-Id: Ie3283738486eb6f8462d594f2728ac98333c0520
Reviewed-on: https://go-review.googlesource.com/20768Reviewed-by: Matthew Dempsky <mdempsky@google.com>