[DebugInfo] Fix incorrect dbg.declare when nrvo flag is used

When clang generates an nrvo boolean flag, the dbg.declare for the
corresponding variable was incorrectly placed on that flag, rather
than the actual variable.

Fix this by not overwriting AllocaAddr with the nrvo flag.

This started causing verifier errors with D158743.

Differential Revision: https://reviews.llvm.org/D158972