[lldb][NFC] Simplify GetLocation_DW_OP_addr function

A very old commit (9422dd64) changed the signature of this function in a
number of ways. This patch aims to improve it:

1. Reword the documentation, which still mentions old parameters that no longer
exist, and to elaborate upon the behavior of this function.
2. Remove the unnecessary parameter `op_addr_idx`. This parameter is odd in a
couple of ways: we never use it with a value that is non-zero, and the matching
`Update_DW_OP_addr` function doesn't use a similar parameter. We also document
that this new behavior. If we ever decide to handle multiple "DW_OP_addr", we
can introduce the complexity again.

Differential Revision: https://reviews.llvm.org/D154265