[ThinLTO] Fix printing of aliases for distributed backend indexes

Summary:
When we import an alias (which will import a copy of the aliasee), but
aren't going to import the aliasee directly, the distributed backend
index will not contain the aliasee summary. Handle this in the summary
assembly printer by printing "null" as the aliasee.

Reviewers: davidxl, dexonsmith

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D48699

llvm-svn: 336160
diff --git a/llvm/test/ThinLTO/X86/distributed_indexes.ll b/llvm/test/ThinLTO/X86/distributed_indexes.ll
index bcde341..371f38f 100644
--- a/llvm/test/ThinLTO/X86/distributed_indexes.ll
+++ b/llvm/test/ThinLTO/X86/distributed_indexes.ll
@@ -40,6 +40,12 @@
 ; BACKEND2-NEXT: <COMBINED_ALIAS
 ; BACKEND2-NEXT: </GLOBALVAL_SUMMARY_BLOCK
 
+; Make sure that when the alias is imported as a copy of the aliasee, but the
+; aliasee is not being imported by itself, that we can still print the summary.
+; The aliasee should be "null".
+; RUN: llvm-dis %t1.bc.thinlto.bc -o - | FileCheck %s --check-prefix=DIS
+; DIS: aliasee: null
+
 declare void @g(...)
 declare void @analias(...)