[llvm] [llvm-exegesis] Simplify validation event string conversion functions (PR #82092)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

It doesn't look like there's an easy way to automatically generate the command line option enum values in `llvm-exegesis.cpp` for the flag definition. If I figure something out I'll open a patch for that as that should reconcile even more mappings.

https://github.com/llvm/llvm-project/pull/82092