Add QTC_GUARD

Similar to QTC_CHECK, but has the boolean result of the condition as
value. Use it in the form

if (QTC_GUARD(condition)) { ... }

Replace some occurrences of

QTC_CHECK(condition)
if (condition) { .... }

Change-Id: I8eea02f869eb7a183dfbf4810dace48097880519
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/cpptools/semantichighlighter.cpp

@@ -103,8 +103,7 @@ void SemanticHighlighter::onHighlighterFinished()
     QTC_ASSERT(m_watcher, return);
     if (!m_watcher->isCanceled() && documentRevision() == m_revision) {
         SyntaxHighlighter *highlighter = m_baseTextDocument->syntaxHighlighter();
-        QTC_CHECK(highlighter);
-        if (highlighter) {
+        if (QTC_GUARD(highlighter)) {
             qCDebug(log) << "onHighlighterFinished() - clearing formats";
             clearExtraAdditionalFormatsUntilEnd(highlighter, m_watcher->future());
         }