[ThinLTO] Handle an external call from an import to an alias in dest

[ThinLTO] Handle an external call from an import to an alias in dest

The findExternalCalls routine ignores calls to functions already
defined in the dest module. This was not handling the case where
the definition in the current module is actually an alias to a
function call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257493 91177308-0d34-0410-b5e6-96231b3b80d8