[ObjC] Support emission of selector stubs calls instead of objc_msgSend. (#183922)

This optimizes objc_msgSend calls by emitting "selector stubs" instead.

Usually, the linker redirects calls to external symbols to a symbol stub
it generates, which loads the target function's address from the GOT and
branches to it:

  <symbol stub for _func:>
    adrp x16, _func@GOTPAGE
    ldr x16, [x16, _func@GOTPAGEOFF]
    br x16

with msgSend selector stubs, we extend that to compute the selector as
well:

  <selector stub for "foo":>
    adrp x1, <selector ref for "foo">@PAGE
    ldr x1, [x1, <selector ref for "foo">@PAGEOFF]
    adrp x16, _objc_msgSend@GOTPAGE
    ldr x16, [x16, _objc_msgSend@GOTPAGEOFF]
    br x16

This lets us avoid loading the selector in the compiler, hopefully
leading to codesize reductions.

We're considering two kinds of stubs, the combined one above with the
objc_msgSend dispatch included, or a shorter one that forwards to the
existing stub for objc_msgSend. That's a decision to be made in the
linker.

Concretely, instead of something like:

  %sel = load i8*, i8** @<selector ref for "foo">
  call ... @objc_msgSend(i8* self, i8* sel)

we emit a call to a newly-declared external "function":

  call ... @"objc_msgSend$foo"(i8* self, i8* undef)

where "objc_msgSend$foo" is treated as any other external symbol
reference throughout the compiler, and, at link-time, is recognized by
the linker as a selector stub.

There are other ways to implement this. An obvious one is to combine
away the objc_msgSend(self, load(sel)) pattern at the IR level.

Both seem feasible, but the IRGen approach might save us a tiny bit of
compile-time, with the assumption that loads need more mid-level
analysis than boring external functions.

This optimization requires linker version 811.2 or newer for arm64,
arm64e, and arm64_32.

rdar://84437635