LowerDbgDeclare: look through bitcasts.

Summary:
Emit a value debug intrinsic (with OP_deref) when an alloca address is
passed to a function call after going through a bitcast.

This generates an FP or SP-relative location for the local variable in
the following case:
  int x;
  use((void *)&x;

Reviewers: aprantl, vsk, pcc

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70752