ToolChain: Change languageId for C++

"C++" might end up clashing with Orgad's work to make variable substitution
more powerful. Use a pure-ASCII-string name (Cxx) instead.

No conversion code since the impact can't be big so far, considering how
new the code is.

Change-Id: Iab1c91cd89d6c11dfaad80472a312e619abb6d97
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/toolchain.cpp

@@ -176,7 +176,7 @@ QString ToolChain::languageId(ToolChain::Language l)
     case Language::C:
         return QStringLiteral("C");
     case Language::Cxx:
-        return QStringLiteral("C++");
+        return QStringLiteral("Cxx");
     };
     return QString();
 }