[FileSystem] Add convenience method to check for directories.

Replace calls to LLVM's is_directory with calls to LLDB's FileSytem
class. For this I introduced a new convenience method that, like the
other methods, takes either a path or filespec. This still uses the LLVM
functions under the hood.

Differential revision: https://reviews.llvm.org/D54135

llvm-svn: 346375