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

[Louis Dionne via llvm-commits]

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@@ -4,6 +4,10 @@
 // MY_RUN: baz
 // MY_LIST: one, two
 // MY_LIST: three, four
+// MY_SPACE_LIST: orange
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ldionne wrote:

We could add a test where we have multiple spaces between items.

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