[clang] [llvm] [Sanitizer] Make sanitizer passes idempotent (PR #99439)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Sanizer passes setting a "no sanitizer" magic variable is backwards.

If this behaviour is the way to go, have clang set a "needs_asan_lowering" or whatever and have the corresponding pass remove it.

It shouldn't be necessary to emit ever increasing lists of pass and target specific cruft in the IR to avoid miscompilation. The opposite way round is much better - compile correctly when the flag is missing, and add the ad hoc metadata to switch on non-default behaviour

https://github.com/llvm/llvm-project/pull/99439