ARM64: track alignment padding registers on AAPCS targets

This implements clause C.8 of the AAPCS in the front-end, so that Clang
accurately knows when the registers run out and it has to insert padding before
the stack objects begin.

PR19432.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206296 91177308-0d34-0410-b5e6-96231b3b80d8