Make sure that if we're going to attempt to add a type to a DIE that

Make sure that if we're going to attempt to add a type to a DIE that
the type exists.

Fix up cases where we weren't checking for optional types and add
an assert to addType to make sure we catch this in the future.

Fix up a testcase that was using the tag for DW_TAG_array_type
when it meant DW_TAG_enumeration_type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187963 91177308-0d34-0410-b5e6-96231b3b80d8