In Parser::SkipUntil do not stop at '@' unconditionally.

Stopping at '@' was originally intended to avoid skipping an '@' at the @interface context
when doing parser recovery, but we should not stop at all '@' tokens because they may be part
of expressions (e.g. in @"string", @selector(), etc.), so in most cases we will want to skip them.

This commit caused 'test/Parser/method-def-in-class.m' to fail for the cases where we tried to
recover from unmatched angle bracket but IMO it is not a big deal to not have good recovery
from such broken code and the way we did recovery would not always work anyway (e.g. if there was '@'
in an expression).

The case that rdar://7029784 is about still passes.

llvm-svn: 146815