[dsymutil] Support and relocate base address selection entries for debug_loc

Since r374600 clang emits base address selection entries. Currently
dsymutil does not support these entries and incorrectly interprets them
as location list entries.

This patch adds support for base address selection entries in dsymutil
and makes sure they are relocated correctly.

Thanks to Dave for coming up with the test case!

Differential revision: https://reviews.llvm.org/D69005

llvm-svn: 374957