compiler-clang.h: Add __diag infrastructure for clang

Add __diag macros similar to those in compiler-gcc.h, so that warnings
that need to be adjusted for specific cases but not globally can be
ignored when building with clang.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220304224645.3677453-6-memxor@gmail.com

[ Kartikeya: wrote commit message ]

include/linux/compiler-clang.h

@@ -68,3 +68,25 @@
 
 #define __nocfi		__attribute__((__no_sanitize__("cfi")))
 #define __cficanonical	__attribute__((__cfi_canonical_jump_table__))
+
+/*
+ * Turn individual warnings and errors on and off locally, depending
+ * on version.
+ */
+#define __diag_clang(version, severity, s) \
+	__diag_clang_ ## version(__diag_clang_ ## severity s)
+
+/* Severity used in pragma directives */
+#define __diag_clang_ignore	ignored
+#define __diag_clang_warn	warning
+#define __diag_clang_error	error
+
+#define __diag_str1(s)		#s
+#define __diag_str(s)		__diag_str1(s)
+#define __diag(s)		_Pragma(__diag_str(clang diagnostic s))
+
+#if CONFIG_CLANG_VERSION >= 110000
+#define __diag_clang_11(s)	__diag(s)
+#else
+#define __diag_clang_11(s)
+#endif