Avoid clearing an empty PrioritizedCXXGlobalInits, NFC

We already test for PrioritizedCXXGlobalInits being non-empty
and process it so it makes sense to clear it only in that if.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240220 91177308-0d34-0410-b5e6-96231b3b80d8