Force Clang modules with LLVM >= 14 (#337)

Otherwise Clang defaults to C++ modules with `-std=c++20`, which breaks
`layering_check`.

Fixes #334

toolchain/cc_toolchain_config.bzl

@@ -40,7 +40,8 @@ def cc_toolchain_config(
         tools_path_prefix,
         wrapper_bin_prefix,
         compiler_configuration,
-        cxx_builtin_include_directories):
+        cxx_builtin_include_directories,
+        major_llvm_version):
     exec_os_arch_key = _os_arch_pair(exec_os, exec_arch)
     target_os_arch_key = _os_arch_pair(target_os, target_arch)
     _check_os_arch_keys([exec_os_arch_key, target_os_arch_key])
@@ -189,6 +190,15 @@ def cc_toolchain_config(
             "-std=" + cxx_standard,
             "-stdlib=libc++",
         ]
+        if major_llvm_version >= 14:
+            # With C++20, Clang defaults to using C++ rather than Clang modules,
+            # which breaks Bazel's `use_module_maps` feature, which is used by
+            # `layering_check`. Since Bazel doesn't support C++ modules yet, it
+            # is safe to disable them globally until the toolchain shipped by
+            # Bazel sets this flag on `use_module_maps`.
+            # https://github.com/llvm/llvm-project/commit/0556138624edf48621dd49a463dbe12e7101f17d
+            cxx_flags.append("-Xclang")
+            cxx_flags.append("-fno-cxx-modules")
         if use_lld:
             # For single-platform builds, we can statically link the bundled
             # libraries.

toolchain/internal/configure.bzl

@@ -372,6 +372,7 @@ cc_toolchain_config(
       "unfiltered_compile_flags": {unfiltered_compile_flags},
     }},
     cxx_builtin_include_directories = {cxx_builtin_include_directories},
+    major_llvm_version = {major_llvm_version},
 )
 
 toolchain(
@@ -541,6 +542,7 @@ cc_toolchain(
             if _is_hermetic_or_exists(rctx, dir, sysroot_path)
         ]),
         extra_compiler_files = ("\"%s\"," % str(toolchain_info.extra_compiler_files)) if toolchain_info.extra_compiler_files else "",
+        major_llvm_version = major_llvm_version,
     )
 
 def _convenience_targets_str(rctx, use_absolute_paths, llvm_dist_rel_path, llvm_dist_label_prefix, exec_dl_ext):