Extended Description

Currently, Darwin constructs the header search paths in cc1. For this to work, it needs information about the C++ STL. This is solved by passing down the -stdlib argument to cc1.

ToolChain::AddClangCXXStdlibIncludeArgs states that this solution is legacy. Instead, Darwin::AddClangCXXStdlibIncludeArgs should be implemented, maybe similar to Linux::AddClangCXXStdlibIncludeArgs.

This also introduces problems with the test Driver/darwin-stdlib.cpp which fails if CLANG_DEFAULT_CXX_STDLIB and will therefore shortly be XFAILed in that case.