Add implicit declarations of allocation functions when looking them up for
redeclaration, not just when looking them up for a use -- we need the implicit
declaration to appropriately check various properties of them (notably, whether
they're deleted).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200729 91177308-0d34-0410-b5e6-96231b3b80d8