GlobalDCE: Delete available_externally initializers if it allows removing the value the initializer is referring to.

This is useful for functions that are not actually available externally but
referenced by a vtable of some kind. Clang emits functions like this for the MS
ABI.

PR20182.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212337 91177308-0d34-0410-b5e6-96231b3b80d8