[LLDB] Fixing DWARFExpression handling of ValueType::FileAddress case for DW_OP_deref_size

Currently DW_OP_deref_size just drops the ValueType::FileAddress case and does
not attempt to handle it. This adds support for this case and a test that
verifies this support.

I did a little refactoring since DW_OP_deref and DW_OP_deref_size have some
overlap in code.

Also see: rdar://66870821

Differential Revision: https://reviews.llvm.org/D121408