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

[Corentin Jabot via cfe-commits]

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@@ -48,6 +48,13 @@ The previous behavior can be restored with `-Wno-error=unicode-whitespace`.
 Clang will stop accepting non-ascii whitespaces as token separators
 in a future version of Clang.
 
+- The experimental ClangIR code generator is now built into Clang by default, so
+`-fclangir` can be used without any additional CMake options. While ClangIR will
+work for many workloads in some narrow configurations (ie, Itanium only), it
+should prove useful for MLIR experimentation in Clang. This does however have a
+measureable and significant impact on the build-time of clang itself. It can be
+disabled with the CMake option `-DCLANG_ENABLE_CIR=Off`.
+
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cor3ntin wrote:

I think this should be under `What's New in Clang` - having in in the LLVM release note is great.
If we think it warrants a more prominent position, lets add a section above `## Potentially Breaking Changes` (`## Clang IR is Enabled by default` or something). In any case this is not "a C/C++ breaking change" (i.e a "your code will be interpreted differently" change).

Also, do you have a link so that people can read about clang ir?

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