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

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b-sumner wrote:

@MaskRay can you talk more about your concerns?  This won't affect compile time, and instrumenting a shadow load will cause an immediate crash of the application at runtime.  This seems much more robust than counting on toolchains to always arrange to avoid double instrumentation.

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