Updating to use more range-based for loops, nullptr and auto. No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206590 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: make sure the caller is expected to extend in AAPCS.

This is one of those DarwinPCS differences. It'd been caught in
arguments, but not return values.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206594 91177308-0d34-0410-b5e6-96231b3b80d8