[DebugInfo] Ignore bitcasts when lowering stack arg dbg.values

Summary:
Look past bitcasts when looking for parameter debug values that are
described by frame-index loads in `EmitFuncArgumentDbgValue()`.

In the attached test case we would be left with an undef `DBG_VALUE`
for the parameter without this patch.

A similar fix was done for parameters passed in registers in D13005.

This fixes PR40777.

Reviewers: aprantl, vsk, jmorse

Reviewed By: aprantl

Subscribers: bjope, javed.absar, jdoerfert, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D58831

llvm-svn: 356363