[llvm] [LLVM][rtsan] Add LLVM nosanitize_realtime attribute (PR #105447)

[Chris Apple via llvm-commits]

cjappl wrote:

> We usually only support `__attribute__((no_sanitize("sanitizername")))` as the attribute for disabling a sanitizer. Some sanitizers have other attributes for disabling them but they only exist for backwards compatibility reasons.

Can you elaborate how one would access this in a transform pass of LLVM? I'm generally new to this area of the code, and thought that adding a nosanitize_x attribute was the only way to accomplish my goal.

The next PR after this one adds a check for this LLVM attribute in RealtimeSanitizer.cpp, and if this attribute exists on the function inserts a function call to bypass this in the Realtime Sanitizer stack. (That looks something like [this](https://github.com/cjappl/llvm-project/pull/4)). This seems plausible to do without the attribute as long as I have the correct thing to look for.

https://github.com/llvm/llvm-project/pull/105447