[clang] [llvm] [coroutine] Implement llvm.coro.await.suspend intrinsic (PR #79712)

[via cfe-commits]

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@@ -1744,6 +1744,273 @@ a call to ``llvm.coro.suspend.retcon`` after resuming abnormally.
 In a yield-once coroutine, it is undefined behavior if the coroutine
 executes a call to ``llvm.coro.suspend.retcon`` after resuming in any way.
 
+.. _coro.await.suspend:
+
+'llvm.coro.await.suspend' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+::
+
+  declare void @llvm.coro.await.suspend(
+                ptr <awaiter>,
+                ptr <handle>,
+                ptr <await_suspend_function>)
+
+Overview:
+"""""""""
+
+The '``llvm.coro.await.suspend``' intrinsic hides C++ `await-suspend`
+block code from optimizations on presplit coroutine body 
+to avoid miscompilations. This version of intrinsic corresponds to 
+'``void awaiter.await_suspend(...)``' variant.
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fpasserby wrote:

Added motivation (copied that comment almost verbatim)

https://github.com/llvm/llvm-project/pull/79712