[clang] [clang-cl] Map /Ot to -O3 instead of -O2 (PR #95406)

[via cfe-commits]

zmodem wrote:

@AaronBallman as discussed on https://discourse.llvm.org/t/clang-cl-optimization-option/79554

Also @nico who IIRC looked at these flags in recent times.

https://github.com/llvm/llvm-project/pull/95406