[clang] [clang][NFC] Create CodeGenShared component for shared code with ClangIR (PR #157936)

[Andy Kaylor via cfe-commits]

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@@ -0,0 +1,117 @@
+//===--- ItaniumRTTIBuilder.h - LLVM Backend Utilities ----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LIB_CODEGENSHARED_ITANIUMRTTIBUILDER_H
+#define LLVM_CLANG_LIB_CODEGENSHARED_ITANIUMRTTIBUILDER_H
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andykaylor wrote:

```suggestion
#ifndef CLANG_CODEGENSHARED_ITANIUMRTTIBUILDER_H
#define CLANG_CODEGENSHARED_ITANIUMRTTIBUILDER_H
```
The overwhelming majority of current clang headers have `LLVM_` as part of their header guard, but that's not consistent with [what the coding standard says](https://llvm.org/docs/CodingStandards.html#header-guard). I think it's best to follow the standard here, but if others prefer consistency, I can go along with that. Either way, this should not include `_LIB`.

https://github.com/llvm/llvm-project/pull/157936