C++: avoid partial duplicate implementation of cp_parser_error

C++: avoid partial duplicate implementation of cp_parser_error

In r251026 (aka 3fe34694f0990d1d649711ede0326497f8a849dc,
"C/C++: show pertinent open token when missing a close token")
I copied part of cp_parser_error into cp_parser_required_error,
leading to duplication of code.

This patch eliminates this duplication by merging the two copies of the
code into a new cp_parser_error_1 subroutine.

Doing so removes an indentation level, making the patch appear to have
more churn than it really does.

The patch also undoes the change to g++.dg/parse/pragma2.C, as the
old behavior is restored.

From-SVN: r253686