Turning IAS on by default on ARM/Thumb

This is an experimental feature, where -integrated-as will be
on by default on ARM/Thumb. We aim to detect the missing features
so that the next release is stable.

Updating the ReleaseNotes, too.

Also moving the AArch64 into the same place.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197024 91177308-0d34-0410-b5e6-96231b3b80d8