Avoid a recursive function call that could run LLDB out of file descriptors in...

Avoid a recursive function call that could run LLDB out of file descriptors in FileSystem::DeleteDirectory(...).

Fixes include:
- use FileSystem::Unlink() instead of a direct call to ::unlink(...) when deleting files when iterating through the current directory
- save directories from current directory in a list and iterate through those _after_ the current directory has been iterated
- Use new FileSpec::ForEachItemInDirectory() instead of manually iterating across directories with opendir()/readdir()/closedir()

We should switch all code over to using FileSpec::ForEachItemInDirectory(...) in the near future and get rid of FileSpec::EnumerateDirectory().

This is a follow up patch to:

http://reviews.llvm.org/D10787

llvm-svn: 240978