Emit Retags in Codegen

[nikomatsakis]

Metadata
Point of contact	@icmccorm
Team(s)	compiler, opsem
Goal document	2025h2/codegen_retags

Summary

Allow codegen backends to implement the MIR Retag intrinsic, and add a similar intrinsic to the LLVM backend.

Tasks and status

• Implementation (@icmccorm)
• Author RFC (@icmccorm)
• Design meeting (opsem, compiler )
• Standard reviews (opsem, compiler )
• Dedicated reviewer (opsem, compiler )
• RFC decision (opsem, compiler )
• Discussion and moral support (opsem, compiler )

[nikomatsakis]

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