C++: Disable C++ keywords in C files

In some (legacy) C files, new and delete may be used for regular identifier.

There are some limitations:
* Header files have no 'implicit' type, and may be parsed as C++ or ObjC depending on the
other files in the project.
* QMakeProject use a single ProjectPart for C and C++ files, so there will still be the issue.

Change-Id: Iec11687b35f7ccf1e7c0d091b143ae90d950e440
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/cplusplus/MatchingText.cpp

@@ -201,6 +201,7 @@ static Tokens getTokens(const QTextCursor &cursor, int &prevState)
     features.qtKeywordsEnabled = false;
     features.qtMocRunEnabled = false;
     features.cxx11Enabled = true;
+    features.cxxEnabled = true;
     features.c99Enabled = true;
 
     SimpleLexer tokenize;