[clang] Multilib support for libraries with exceptions (PR #75031)

[Michael Platings via cfe-commits]

https://github.com/mplatings commented:

I'd be disinclined to add `CachedExceptionsMode`:
* It's more code to maintain.
* Most toolchains will never use it so for them it's a small extra runtime cost with no benefit.

I think you could calculate the exceptions mode in `getMultilibFlags` when needed.

I'm not actively working on LLVM any more so I'm going to remove myself as reviewer now.

https://github.com/llvm/llvm-project/pull/75031