[AArch64] Define subtarget feature "reserve-x18", which is used to decide

whether register x18 should be reserved.

This change is needed because we cannot use a backend option to set
cl::opt "aarch64-reserve-x18" when doing LTO.

Out-of-tree projects currently using cl::opt option "-aarch64-reserve-x18"
to reserve x18 should make changes to add subtarget feature "reserve-x18"
to the IR.

rdar://problem/21529937

Differential Revision: http://reviews.llvm.org/D11463

llvm-svn: 243186