[llvm] [LLVM][CMake][NFC] Use generator expression to separate CXXFLAGS (PR #173869)

[Alexis Engelke via llvm-commits]

aengelke wrote:

A tangentially related questions is whether `llvm-config --cxxflags` should actually include `-fno-exceptions -funwind-tables`, because IIUC these is not required to link against LLVM, only `-fno-rtti` is. So I think the fix is to add the RTTI flag separately, as this is the only important flag that we need to emit?

https://github.com/llvm/llvm-project/pull/173869