[libc++] Automatically detect the libc++ hardening mode from the test suite (#172505)

This prevents hardcoding the hardening mode via compiler flags, and
allows testing what the default hardening mode is on platforms that set
it to something that isn't `none`. Otherwise, a platform setting a
default (which is done via -DLIBCXX_HARDENING_MODE=mode at CMake
configuration time) would end up passing `-D_LIBCPP_HARDENING_MODE=mode`
to the compiler, which does not allow checking what the default mode is.

libcxx/test/CMakeLists.txt

@@ -41,8 +41,6 @@ if (NOT LIBCXX_ENABLE_RTTI)
   serialize_lit_param(SERIALIZED_LIT_PARAMS enable_rtti False)
 endif()
 
-serialize_lit_string_param(SERIALIZED_LIT_PARAMS hardening_mode "${LIBCXX_HARDENING_MODE}")
-
 if (CMAKE_CXX_COMPILER_TARGET)
   serialize_lit_string_param(SERIALIZED_LIT_PARAMS target_triple "${CMAKE_CXX_COMPILER_TARGET}")
 else()

libcxx/utils/libcxx/test/features/libcxx_macros.py

@@ -6,7 +6,7 @@
 #
 # ===----------------------------------------------------------------------===##
 
-from libcxx.test.dsl import Feature, compilerMacros
+from libcxx.test.dsl import Feature, compilerMacros, programSucceeds
 
 features = []
 
@@ -74,3 +74,22 @@ for macro, feature in inverted_macros.items():
             and compilerMacros(cfg)[m] == "0",
         )
     )
+
+for mode in ("none", "fast", "extensive", "debug"):
+    check_program = f"""
+        #include <stddef.h> // any header to get the definitions
+        int main(int, char**) {{
+        #if defined(_LIBCPP_VERSION) && \\
+                defined(_LIBCPP_HARDENING_MODE) && _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_{mode.upper()}
+            return 0;
+        #else
+            return 1;
+        #endif
+        }}
+    """
+    features.append(
+        Feature(
+            name=f"libcpp-hardening-mode={mode}",
+            when=lambda cfg, prog=check_program: programSucceeds(cfg, prog)
+        )
+    )

libcxx/utils/libcxx/test/params.py

@@ -420,7 +420,6 @@ DEFAULT_PARAMETERS = [
                 AddCompileFlag("-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST")      if hardening_mode == "fast" else None,
                 AddCompileFlag("-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE") if hardening_mode == "extensive" else None,
                 AddCompileFlag("-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG")     if hardening_mode == "debug" else None,
-                AddFeature("libcpp-hardening-mode={}".format(hardening_mode))               if hardening_mode != "undefined" else None,
             ],
         ),
     ),