[clang] [Fuchsia] Explicitly set C++ standard C++23 in libcxx tests (PR #73760)

[Petr Hosek via cfe-commits]

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@@ -28,6 +28,7 @@ set(LLVM_STATIC_LINK_CXX_STDLIB ON CACHE BOOL "")
 set(LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
 set(LLDB_ENABLE_CURSES OFF CACHE BOOL "")
 set(LLDB_ENABLE_LIBEDIT OFF CACHE BOOL "")
+set(LIBCXX_TEST_PARAMS "std=c++23" CACHE STRING "")
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petrhosek wrote:

This needs to be set separately for all targets, i.e. `RUNTIMES_${target}_LIBCXX_TEST_PARAMS`, `LIBCXX_TEST_PARAMS` should be only set for Darwin.

https://github.com/llvm/llvm-project/pull/73760