C++: fix digraph parsing for <:: exception.

According to section 2.5 from the standard:
""" If the input stream has been parsed into preprocessing tokens up to
a given character: ... Otherwise, if the next three characters are <::
and the subsequent character is neither : nor >, the < is treated as a
preprocessor token by itself and not as the first character of the
alternative token <:. """

Change-Id: Ib9cdac61e3c2243d1bc1d4471a09ae6bd839fdda
Task-number: QTCREATORBUG-13253
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>