Clang*: Workaround parsing mingw's <type_traits> with clang 3.9

...for -std=gnu++0x.

Define the unknown identifier/type "__float128" so at least the headers can be
parsed successfully. Of course this does not help for parsing client code
referencing that identifier, but this should be the less common case.

Task-number: QTCREATORBUG-17126
Change-Id: Id321311713029d8aa77e068b02361d86debfada6
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -368,6 +368,14 @@ void CompilerOptionsBuilder::undefineCppLanguageFeatureMacrosForMsvc2015()
     }
 }
 
+void CompilerOptionsBuilder::addDefineFloat128ForMingw()
+{
+    // TODO: Remove once this is fixed in clang >= 3.9.
+    // https://llvm.org/bugs/show_bug.cgi?id=30685
+    if (m_projectPart.toolchainType == ProjectExplorer::Constants::MINGW_TOOLCHAIN_TYPEID)
+        addDefine("#define __float128 void");
+}
+
 QString CompilerOptionsBuilder::includeOption() const
 {
     return QLatin1String("-I");