[LLDB][GUI] Resolve paths in file/directory fields

This patch resolves the paths in the file/directory fields before
performing checks. Those checks are applied on the file system if
m_need_to_exist is true, so remote files can set this to false to avoid
performing host-side file system checks. Additionally, methods to get
a resolved and a direct file specs were added to be used by client code.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D106553