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

[Nikita Popov via llvm-commits]

nikic wrote:

@krasimirgg Rust shouldn't need LLVM built with RTTI. I'm not sure how this PR changed things. It looks like https://github.com/rust-lang/rust/blob/main/compiler/rustc_llvm/build.rs doesn't set `-fno-exceptions` or `-fno-rtti`, but the LLVM build also doesn't change the defaults (exceptions and RTTI disabled), so it's not clear to me how it worked before...

It would be good to get a full build log with the compiler invocations, so the used flags are visible.

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