C++: Support for UTF-8 in the lexer

This will save us toLatin1() conversations in CppTools (which already
holds UTF-8 encoded QByteArrays) and thus loss of information (see
QTCREATORBUG-7356). It also gives us support for non-latin1 identifiers.

API-wise the following functions are added to Token. In follow-up
patches these will become handy in combination with QStrings.
    utf16chars() - aequivalent of bytes()
    utf16charsBegin() - aequivalent of bytesBegin()
    utf16charsEnd() - aequivalent of bytesEnd()

Next steps:
 * Adapt functions from TranslationUnit. They should work with utf16
   chars in order to calculate lines and columns correctly also for
   UTF-8 multi-byte code points.
 * Adapt the higher level clients:
    * Cpp{Tools,Editor} should expect UTF-8 encoded Literals.
    * Cpp{Tools,Editor}: When dealing with identifiers on the
      QString/QTextDocument layer, code points
      represendet by two QChars need to be respected, too.
 * Ensure Macro::offsets() and Document::MacroUse::{begin,end}() report
   offsets usable in CppEditor/CppTools.

Addresses QTCREATORBUG-7356.

Change-Id: I0791b5236be8215d24fb8e38a1f7cb0d279454c0
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
C++: TranslationUnit::getPosition takes utf16char offsets

...and not byte offsets anymore. This is necessary in order to calculate
the line and column numbers correctly with respect to unicode code
points.

Change-Id: I5d79857b3eaefeb8d563b4f1e3938a64debc5e08
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
C++/CppTools: Pass UTF-8 encoded source to Lexer

The Lexer can handle it now.

Task-number: QTCREATORBUG-7356
Change-Id: I8c4b03a247656e013d44c3cedca4835e133d4036
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
C++: Use Token::utf16chars{Begin,End} where appropriate

...especially in CppTools/CppEditor where the offsets are used with a
QString/QTextDocument.

Change-Id: Ic6d18fbc01fb9cc899a9bd2d7424cd2edae487f1
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Cpp{Tools,Editor}: Tests: Use QString instead of QByteArray

This is necessary in order to add tests with multi-byte UTF-8 code
points. Otherwise the initial and target source code marker positions
will be calculated on the QByteArray (test code) but used with a QString
(editor document).

Change-Id: I108961b13d32912a4d3193cf26eb59f65d296f57
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>