Trigraphs must only be parsed before/during preprocessing. The preprocessor
will now replace trigraphs with their standard form, and re-lexing in
TranslationUnit will not try to parse any trigraph.

Also added a few missing trigraphs: ??=, ??', ??! and ??-.

Task-number: QTCREATORBUG-13253
Change-Id: I1723ed53b00090b878c22b83b7e963b647b65f72
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>