[LLDB] - Improve the support of .debug_str_offsets/.debug_str_offsets.dwo

A skeleton compilation unit may contain the DW_AT_str_offsets_base attribute
that points to the first string offset of the CU contribution to the
.debug_str_offsets. At the same time, when we use split dwarf,
the corresponding split debug unit also
may use DW_FORM_strx* forms pointing to its own .debug_str_offsets.dwo.
In that case, DWO does not contain DW_AT_str_offsets_base, but LLDB
still need to know and skip the .debug_str_offsets.dwo section header to
access the offsets.

The patch implements the support of DW_AT_str_offsets_base.

Differential revision: https://reviews.llvm.org/D54844

llvm-svn: 347859