Add hack (provided by Jonathan Sauer) to fall back to assuming Xcode is installed in /Developer

when using Python < 2.7.0.  This is the case on Snow Leopard, where the tools are always
installed in /Developer.  This isn't a proper fix for really figuring out where Xcode
is installed, but should work to fix an obvious problem on Snow Leopard.

llvm-svn: 160321