Clang: never include a define for __cplusplus.

The flags used by the project explorer to interrogate the toolchain for
compiler-defined #defines always indicate a C++ file. So, this will
always include a #define for __cplusplus. When editing C code with
the clang code model, this results in extern "C" linkage specifications,
which result in warnings or errors.

The proper fix is described in QTCREATORBUG-11709, but is too big to be
done within the 3.1 timeframe.

Task-number: QTCREATORBUG-11501
Change-Id: Id9e261fa8d429fead4a2cd5fd7398aa6e1e8c13d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/clangcodemodel/clangutils.cpp

@@ -113,6 +113,11 @@ static QStringList buildDefines(const QByteArray &defines, bool toolchainDefines
         if (def.isEmpty())
             continue;
 
+        // This is a quick fix for QTCREATORBUG-11501.
+        // TODO: do a proper fix, see QTCREATORBUG-11709.
+        if (def.startsWith("#define __cplusplus"))
+            continue;
+
         // TODO: verify if we can pass compiler-defined macros when also passing -undef.
         if (toolchainDefines) {
             //### FIXME: the next 3 check shouldn't be needed: we probably don't want to get the compiler-defined defines in.