[libc++] Fix module_std_compat.sh.cpp with new versions of clang-tidy (llvm#170446)

clang-tidy doesn't run checks anymore in system headers by default in
new versions to reduce runtime. In this case it's problematic, since we
want to check system headers.

Author: philnik777

libcxx/utils/libcxx/test/modules.py

@@ -166,7 +166,7 @@ def process_module_partition(self, header, is_c_header):
             f'" > {self.tmp_prefix}.{header}.cppm'
         )
 
-        # Extract the information of the module partition using lang-tidy
+        # Extract the information of the module partition using clang-tidy
         print(
             f"// RUN: {self.clang_tidy} {self.tmp_prefix}.{header}.cppm "
             "  --checks='-*,libcpp-header-exportable-declarations' "
@@ -222,6 +222,7 @@ def process_header(self, header, include, is_c_header):
         print(f'// RUN: echo -e "' f"{include}" f'" > {self.tmp_prefix}.{header}.cpp')
         print(
             f"// RUN: {self.clang_tidy} {self.tmp_prefix}.{header}.cpp "
+            "  --system-headers "
             "  --checks='-*,libcpp-header-exportable-declarations' "
             "  -config='{CheckOptions: [ "
             "    {"