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

[via llvm-commits]

GkvJwa 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?

Sure, we can overwrite RELWITHDEBINFO based on the CMAKE_CXX_FLAGS_RELEASE flags like this
```
  set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELEASE} /Zi")
  set(CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_EXE_LINKER_FLAGS_RELEASE} /DEBUG /OPT:REF /OPT:ICF")
  set(CMAKE_SHARED_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_SHARED_LINKER_FLAGS_RELEASE} /DEBUG /OPT:REF /OPT:ICF")
```
and do we need to add some additional flags for FLAGS_RELEASE, such as /FIXED:NO

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