[lldb] 2/2: Fix DW_AT_ranges DW_FORM_sec_offset not using DW_AT_rnglists_base (used by GCC)

DW_AT_ranges can use DW_FORM_sec_offset (instead of DW_FORM_rnglistx).
In such case DW_AT_rnglists_base does not need to be present.
DWARF-5 spec:
        "If the offset_entry_count is zero, then DW_FORM_rnglistx cannot
        be used to access a range list; DW_FORM_sec_offset must be used
        instead. If the offset_entry_count is non-zero, then
        DW_FORM_rnglistx may be used to access a range list;"

This fix is for TestTypeCompletion.py category `dwarf` using GCC with DWARF-5.

The fix just provides GetRnglist() lazy getter for `m_rnglist_table`.
The testcase is easier to review by:
        diff -u lldb/test/Shell/SymbolFile/DWARF/DW_AT_low_pc-addrx.s \
          lldb/test/Shell/SymbolFile/DWARF/DW_AT_range-DW_FORM_sec_offset.s

Differential Revision: https://reviews.llvm.org/D98289