[clang] [CIR][docs] C++ cleanup and exception handling design for CIR (PR #177625)

[Erich Keane via cfe-commits]

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+=============================================
+ClangIR Cleanup and Exception Handling Design
+=============================================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+This document describes the proposed new design for C++ cleanups and exception
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erichkeane wrote:

I guess I just presumed that our way of generating (create alloca, create ctor call, enter cleanup scope) 1 after another would end up with the allocas inside the scope, though hoistable. 

OR are we going to have some additional infrastructure to put the allocas at the top?

https://github.com/llvm/llvm-project/pull/177625