[lldb/DWARF] Add support for pre-standard GNU call site attributes

Summary:
The code changes are very straight-forward -- just handle both DW_AT_GNU
and DW_AT_call versions of all tags and attributes. There is just one
small gotcha: in the GNU version, DW_AT_low_pc was used both for the
"return pc" and the "call pc" values, depending on whether the tag was
describing a tail call, while the official scheme uses different
attributes for the two things.

Reviewers: vsk, dblaikie

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D80519