[ThinLTO] Always set linkage type to external when converting alias

When converting an alias to a non-alias when the aliasee is not
imported, ensure that the linkage type is set to external so that it is
a valid linkage type. Added a test case that exposed this issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252054 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed