[clang] [clang][docs] Add documentation for EH codegen (PR #176236)

[Eli Friedman via cfe-commits]

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+========================================
+LLVM IR Generation for EH and Cleanups
+========================================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+This document describes how Clang's LLVM IR generation represents exception
+handling (EH) and C++ cleanups. It focuses on the data structures and control
+flow patterns used to model normal and exceptional exits, and it outlines how
+the generated IR differs across common ABI models.
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efriedma-quic wrote:

Probably should cross-reference LLVM EH documentation somewhere.

https://github.com/llvm/llvm-project/pull/176236