[libc++] Properly detect whether C++20 modules are supported by the compiler (#94275)

It is possible for a compiler to support the warning without being able
to compile `export module foo;`, so use that in addition to the warning
to check whether C++20 modules are supported.

libcxx/utils/libcxx/test/features.py

@@ -317,7 +317,14 @@ DEFAULT_FEATURES = [
         or ("_WIN32" in compilerMacros(cfg) and not _mingwSupportsModules(cfg))
         or platform.system().lower().startswith("aix")
         # Avoid building on platforms that don't support modules properly.
-        or not hasCompileFlag(cfg, "-Wno-reserved-module-identifier"),
+        or not hasCompileFlag(cfg, "-Wno-reserved-module-identifier")
+        or not sourceBuilds(
+            cfg,
+            """
+            export module test;
+            int main(int, char**) { return 0; }
+          """,
+        ),
     ),
     # The time zone validation tests compare the output of zdump against the
     # output generated by <chrono>'s time zone support.