[AArch64] Always add -tune-cpu argument to -cc1 driver

This patch ensures that we always tune for a given CPU on AArch64
targets when the user specifies the "-mtune=xyz" flag. In the
AArch64Subtarget if the tune flag is unset we use the CPU value
instead.

I've updated the release notes here:

  llvm/docs/ReleaseNotes.rst

and added tests here:

  clang/test/Driver/aarch64-mtune.c

Differential Revision: https://reviews.llvm.org/D110258