In Dwarf 3 (and Dwarf 2) attributes whose value are offsets into a
section use the form DW_FORM_data4 whilst in Dwarf 4 and later they
use the form DW_FORM_sec_offset.

This patch updates the places where such attributes are generated to
use the appropriate form depending on the Dwarf version. The DIE entries
affected have the following tags:
DW_AT_stmt_list, DW_AT_ranges, DW_AT_location, DW_AT_GNU_pubnames,
DW_AT_GNU_pubtypes, DW_AT_GNU_addr_base, DW_AT_GNU_ranges_base

It also adds a hidden command line option "--dwarf-version=<uint>"
to llc which allows the version of Dwarf to be generated to override
what is specified in the metadata; this makes it possible to update
existing tests to check the debugging information generated for both
Dwarf 4 (the default) and Dwarf 3 using the same metadata.

Patch (slightly modified) by Keith Walker!