[ThinLTO] Ensure function summary output order is stable
[ThinLTO] Ensure function summary output order is stable

Iterate over the function list instead of a DenseMap of Function pointers
when emitting the function summary into the module.

This fixes PR26419.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@259398 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm