DWARFExpression: Fix implementation of DW_OP_pick

Summary:
The DWARF spec states that the DWARF stack arguments are numbered from
the top. Our implementation of DW_OP_pick was counting them from the
bottom.

This bug probably wasn't noticed because nobody (except my upcoming
postfix-to-DWARF converter) uses DW_OP_pick, but I've cross-checked with
gdb to confirm that counting from the top is the expected behavior.

This patch fixes the implementation to match the spec and gdb behavior
and adds a test.

Reviewers: jasonmolenda, clayborg

Subscribers: mgorny, aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D61182

llvm-svn: 359436