[lldb] Introduce FileSpec::GetComponents

This patch introduces FileSpec::GetComponents, a method that splits a
FileSpec's path into its individual components. For example, given
/foo/bar/baz, you'll get back a vector of strings {"foo", "bar", baz"}.

The motivation here is to reduce the use of
`FileSpec::RemoveLastPathComponent`. Mutating a FileSpec is expensive,
so providing a way of doing this without mutation is useful.

Differential Revision: https://reviews.llvm.org/D151399