ARM64: initial clang support commit.

This adds Clang support for the ARM64 backend. There are definitely
still some rough edges, so please bring up any issues you see with
this patch.

As with the LLVM commit though, we think it'll be more useful for
merging with AArch64 from within the tree.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205100 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGen: Allow different RTTI emission strategies

Some ABIs and C++ libraries may make different trade-offs in how RTTI
is emitted (currently with respect to visibility and so on). This
implements one scheme, as used by ARM64.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205101 91177308-0d34-0410-b5e6-96231b3b80d8