Added a file descriptor interposing library for darwin. This can catch all

functions that can create file descriptors and close them. It will warn when
there close file descriptor call that returns with EBADF and show the 
corresponding stack backtraces that caused the issue. It will also log all
file descriptor create and delete calls. See the comments at the top of
FDInterposing.cpp for all of the details.

llvm-svn: 147816