DIBuilder: Add function and method definitions to the list of all subprograms

Previously we seemed to be assuming that all functions were definitions and all
methods were declarations. This may be consistent with how Clang uses DIBuilder
but doesn't have to be true of all clients (such as DragonEgg).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175423 91177308-0d34-0410-b5e6-96231b3b80d8