[clang] [llvm] Enable ClangIR Build By Default (PR #221291)

[Aaron Ballman via cfe-commits]

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@@ -226,6 +226,11 @@ Makes programs 10x faster by doing Special New Thing.
   found with an explanatory message instead of succeeding with no targets;
   such a build never provided a usable MLIR SDK.
 
+* When Clang is in the `LLVM_ENABLE_PROJECTS` list, it now compiles the
+  experimental MLIR-Generator ClangIR by default. This functionality can
+  increase the compile time of Clang significantly.  This can be disabled with
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AaronBallman wrote:

Maybe mention the size increase here as well?

https://github.com/llvm/llvm-project/pull/221291