C++: fix multi-byte character handling in input.

Temporary fix: if a single byte is found with the highest bit set, then
convert from utf8 to latin1. This can be removed when the lexer can
handle multi-byte characters.

Task-number: QTCREATORBUG-10141
Change-Id: I36a17aa18bd1b2378f12d0cecf4fd4957b38d8f2
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>