CodeGen: use # as the comment leader for ARC marker

Use # as the comment leader for AArch64 auto-release elision marker.
This is to keep it in sync with the value used in swift.  When building
libdispatch for Linux AArch64, the auto-release elision marker was
emitted.  However, ELF uses # as the comment leader while MachO accepts
both ; and #.  Use the common marker for it instead.

llvm-svn: 294877