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>

tests/auto/cplusplus/semantic/tst_semantic.cpp

@@ -142,6 +142,7 @@ public:
         features.qtMocRunEnabled = qtMocRun;
         features.qtKeywordsEnabled = qtMocRun;
         features.cxx11Enabled = enableCxx11;
+        features.cxxEnabled = true;
         diag.errorCount = 0; // reset the error count.
         TranslationUnit *unit = parse(source, TranslationUnit::ParseTranlationUnit, features);
         QSharedPointer<Document> doc(new Document(unit));