[DWARF] Fix referencing Range List Tables from CUs for DWARF64.

As DW_AT_rnglists_base points after the header and headers have
different sizes for DWARF32 and DWARF64, we have to use the format
of the CU to adjust the offset correctly in order to extract
the referenced range list table.

The patch also changes the type of RangeSectionBase because in DWARF64
it is 8-bytes long.

Differential Revision: https://reviews.llvm.org/D67098

llvm-svn: 371016