IRGen: Add metadata for async funclets denoting frame entry and frame exists

Adds sections `__TEXT,__swift_as_entry`, and `__TEXT,__swift_as_ret` that
contain relative pointers to async functlets modelling async function entries,
and function returns, respectively.

Emission of the sections can be trigger with the frontend option
`-Xfrontend -enable-async-frame-push-pop-metadata`.

This is done by:

* IRGen adding a `async_entry` function attribute to async functions.
* LLVM's coroutine splitting identifying continuation funclets that
  model the return from an async function call by adding the function
  attribute `async_ret`.  (see #llvm-project/pull/9204)
* An LLVM pass that keys off these two function attribute and emits the
  metadata into the above mention sections.

rdar://134460666

lib/Frontend/CompilerInvocation.cpp

@@ -3435,6 +3435,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
       Args.hasFlag(OPT_enable_split_cold_code,
                    OPT_disable_split_cold_code,
                    Opts.EnableHotColdSplit);
+  Opts.EmitAsyncFramePushPopMetadata =
+    Args.hasFlag(OPT_enable_async_frame_push_pop_metadata,
+                 OPT_disable_async_frame_push_pop_metadata,
+                 Opts.EmitAsyncFramePushPopMetadata);
   Opts.EnableLargeLoadableTypesReg2Mem =
       Args.hasFlag(OPT_enable_large_loadable_types_reg2mem,
                    OPT_disable_large_loadable_types_reg2mem,