ADT: Correct APInt::getActiveWords for zero values

PR15138 was opened because of a segfault in the Bitcode writer.
The actual issue ended up being a bug in APInt where calls to
APInt::getActiveWords returns a bogus value when the APInt value
is 0.  This patch fixes the problem by ensuring that getActiveWords
returns 1 for 0 valued APInts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174641 91177308-0d34-0410-b5e6-96231b3b80d8