[libcxx] [llvm] [libc++][test] `ADDITIONAL_COMPILE_FLAGS` should be a space-separated list (PR #73541)

[Stephan T. Lavavej via llvm-commits]

StephanTLavavej wrote:

@ldionne @mordante @philnik777 I've reverted the original approach, then implemented space-separated parsing and changed all occurrences of commas in the codebase.

https://github.com/llvm/llvm-project/pull/73541