Decr pc when looking for DWARF loc list entry mid-stack

When looking for a variable location in a DWARF location list,
we search the list of ranges to find one that includes the pc.
With a function mid-stack, the "pc" is the return pc instead of
the call instruction, and in optimized code this can be another
function or a different basic block (with different variable
locations).  Back up the "pc" value mid-stack to find the correct
location list entry.

Differential Revision: https://reviews.llvm.org/D124597
rdar://63903416