[clang] [llvm] Reimplement constrained 'trunc' using operand bundles (PR #118253)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> If you mean an attribute of a call site, then yes, we need more detailed view on side effects in non-default environment. Anyway, performance of a program running with non-default rounding mode should not drop if exception tracking is not needed. As for attribute of an intrinsic, its purpose seems unclear.

I mean a general attribute that can apply to the declaration, and a call site. We need to be able to mark which intrinisic declarations do not care about errno or other fp mode bits, and whether they can read or write them. Furthermore, it is useful to mark individual callsites with stricter variants, just like for memory attributes. We need this to avoid stripping IntrNoMem, it should still be IntrNoMem, with the additional qualifier that strictfp may read/write errno/rounding mode.

https://github.com/llvm/llvm-project/pull/118253