Move AArch64TargetLowering to AArch64Subtarget.
This currently necessitates a TargetMachine for the TargetLowering
constructor and TLOF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210605 91177308-0d34-0410-b5e6-96231b3b80d8
Use unique_ptr for X86Subtarget pointer members.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210606 91177308-0d34-0410-b5e6-96231b3b80d8