DWARFExpression: Fix/add support for (v4) debug_loc base address selection entries

The DWARFExpression is parsing the location lists in about five places.
Of those, only one actually had proper support for base address
selection entries.

Since r374600, llvm has started to produce location expressions with
base address selection entries more aggresively, which caused some tests
to fail.

This patch adds support for these entries to the places which had it
missing, fixing the failing tests. It also adds a targeted test for the
two of the three fixes, which should continue testing this functionality
even if the llvm output changes. I am not aware of a way to write a
targeted test for the third fix (DWARFExpression::Evaluate).

llvm-svn: 374769