[clang] [NFC][clang] Split clang/lib/CodeGen/CGBuiltin.cpp into target-specific files (PR #132252)

[Benjamin Maxwell via cfe-commits]

================
@@ -0,0 +1,8530 @@
+//===------ AArch64.cpp - Emit LLVM Code for builtins ---------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This contains code to emit Builtin calls as LLVM code.
+//
+//===----------------------------------------------------------------------===//
+
----------------
MacDue wrote:

Sure, no worries :+1:  (just an observation -- not blocking this PR).  It may be worth noting that this file contains some ARM builtins too in the comment at the top, just to make it clear they're slightly intertwined. 

Eg. at the:
> This contains code to emit Builtin calls as LLVM code.

https://github.com/llvm/llvm-project/pull/132252