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/ast/tst_ast.cpp

@@ -51,6 +51,7 @@ public:
         const StringLiteral *fileId = control.stringLiteral("<stdin>");
         LanguageFeatures features;
         features.cxx11Enabled = cxx11Enabled;
+        features.cxxEnabled = true;
         features.objCEnabled = true;
         features.qtEnabled = qtMocRun;
         features.qtKeywordsEnabled = qtMocRun;