[DebugInfo] Don't emit incorrect descriptions of thunk params (PR42627)

The `this` parameter of a thunk requires adjustment. Stop emitting an
incorrect dbg.declare pointing to the unadjusted pointer.

We could describe the adjusted value instead, but there may not be much
benefit in doing so as users tend not to debug thunks.

Robert O'Callahan reports that this matches gcc's behavior.

Fixes PR42627.

Differential Revision: https://reviews.llvm.org/D65035

llvm-svn: 367269