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

[Nico Weber via cfe-commits]

nico wrote:

Can you add something like "use /O2 /clang:-O2 to restore the previous behavior" to the commit message, on case someone prefers that?

Otherwise LG.

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