Add support for DW_AT_ranges_base attribute

It is a new attribute emitted by clang as a GNU extension and will
be part of Dwarf5. The purpose of the attribute is to specify a compile
unit level base value for all DW_AT_ranges to reduce the number of
relocations have to be done by the linker.

Fixes (at least partially): https://llvm.org/pr28826

Differential revision: https://reviews.llvm.org/D24514

llvm-svn: 281595