[SelectionDAG] Support multiple dangling debug info for one value

Summary:
When building the selection DAG we sometimes need to postpone
the handling of a dbg.value until the value it should refer to
is created. This is done by using the DanglingDebugInfoMap.
In the past this map has been limited to hold one dangling
dbg.value per value. This patch removes that restriction.

Reviewers: aprantl, rnk, probinson, vsk

Reviewed By: aprantl

Subscribers: Ka-Ka, llvm-commits, JDevlieghere

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D44610

llvm-svn: 328084