Add a few new methods to FileSpec to make it a little easier to work

with directories, without increasing the size of the FileSpec object.
GetPath() returns a std::string of the full pathname of the file.
IsDirectory(), IsPipe(), IsRegularFile(), IsSocket(), and IsSymbolicLink()
can be used instead of getting the FileType() and comparing it to an enum.

Update PlatformDarwinKernel to use these new methods.

llvm-svn: 180704