[NFC] pull a function into its own lambda

[NFC] pull a function into its own lambda

As requested in D46858, pulling this function into its own lambda makes it
easier to read that part of the code and reason as to what's going on because
the scope it can be called from is extremely limited. We want to keep it as a
function because it's called from the two subsequent lines.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332325 91177308-0d34-0410-b5e6-96231b3b80d8