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

[Stephan T. Lavavej via llvm-commits]

================
@@ -1352,7 +1352,7 @@ def getDefaultSubstitutions(test, tmpDir, tmpBase, normalize_slashes=False):
 
     substitutions.append(("%{pathsep}", os.pathsep))
     substitutions.append(("%basename_t", baseName))
-    
+
----------------
StephanTLavavej wrote:

Oh, my editor is configured to strip trailing whitespace on save. Would you like me to undo this line?

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