[DebugInfo] Move the findDbg* functions into DebugInfo.cpp

Move the findDbg* functions into lib/IR/DebugInfo.cpp from
lib/Transforms/Utils/Local.cpp.

D99169 adds a call to a function (findDbgUsers) that lives in
lib/Transforms/Utils/Local.cpp (LLVMTransformUtils) from lib/IR/Value.cpp
(LLVMCore). The Core lib doesn't include TransformUtils. The builtbots caught
this here: https://lab.llvm.org/buildbot/#/builders/109/builds/12664. This patch
moves the function, and the 3 similar ones for consistency, into DebugInfo.cpp
which is part of LLVMCore.

Reviewed By: dblaikie, rnk

Differential Revision: https://reviews.llvm.org/D100632