[llvm] [cmake] Optimize RelWithDebInfo compiler options on windows (PR #101533)

[via llvm-commits]

zmodem wrote:

This patch seems brittle though. What if LLVM's regular Release flags change, how do we ensure that CMAKE_CXX_FLAGS_RELWITHDEBINFO is kept in sync?

Could we define CMAKE_CXX_FLAGS_RELWITHDEBINFO to be CMAKE_CXX_FLAGS_RELEASE + debug flags somewhere?

https://github.com/llvm/llvm-project/pull/101533