cmd/compile: fix bug in logic for computing var abstract origins
The DWARF inline info generation code was using file/line/column (from
src.Pos) as a means of matching up pre- and post-optimization variable
nodes. This turns out to be problematic since it looks as though
distinct formals on the same line can be assigned the same column
number. Work around this issue by adding variable names to the
disambiguation code. Added a testpoint to the linker DWARF test that
checks to make sure each abstract origin offset of distinct within a
given DWARF DW_AT_inlined_routine body.