hardening: Require clang 20.1.0 for __counted_by

After an innocuous change in -next that modified a structure that
contains __counted_by, clang-19 start crashing when building certain
files in drivers/gpu/drm/xe. When assertions are enabled, the more
descriptive failure is:

  clang: clang/lib/AST/RecordLayoutBuilder.cpp:3335: const ASTRecordLayout &clang::ASTContext::getASTRecordLayout(const RecordDecl *) const: Assertion `D && "Cannot get layout of forward declarations!"' failed.

According to a reverse bisect, a tangential change to the LLVM IR
generation phase of clang during the LLVM 20 development cycle [1]
resolves this problem. Bump the version of clang that enables
CONFIG_CC_HAS_COUNTED_BY to 20.1.0 to ensure that this issue cannot be
hit.

Link: llvm/llvm-project@160fb11 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20250807-fix-counted_by-clang-19-v1-1-902c86c1d515@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: nathanchance

init/Kconfig

@@ -117,10 +117,11 @@ config CC_HAS_NO_PROFILE_FN_ATTR
 
 config CC_HAS_COUNTED_BY
 	bool
-	# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
-	# https://github.com/llvm/llvm-project/pull/110497
-	# https://github.com/llvm/llvm-project/pull/112636
-	default y if CC_IS_CLANG && CLANG_VERSION >= 190103
+	# clang needs to be at least 20.1.0 to avoid potential crashes
+	# when building structures that contain __counted_by
+	# https://github.com/ClangBuiltLinux/linux/issues/2114
+	# https://github.com/llvm/llvm-project/commit/160fb1121cdf703c3ef5e61fb26c5659eb581489
+	default y if CC_IS_CLANG && CLANG_VERSION >= 200100
 	# supported since gcc 15.1.0
 	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
 	default y if CC_IS_GCC && GCC_VERSION >= 150100