selftests/bpf: List newest Clang built-ins needed for some CO-RE selftests

Record which built-ins are optional and needed for some of recent BPF CO-RE
subtests. Document Clang diff that fixed corner-case issue with
__builtin_btf_type_id().

Suggested-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/README.rst

···
   https://reviews.llvm.org/D78466
 has been pushed to llvm 10.x release branch and will be
 available in 10.0.1. The fix is available in llvm 11.0.0 trunk.
+
+BPF CO-RE-based tests and Clang version
+=======================================
+
+A set of selftests use BPF target-specific built-ins, which might require
+bleeding-edge Clang versions (Clang 12 nightly at this time).
+
+Few sub-tests of core_reloc test suit (part of test_progs test runner) require
+the following built-ins, listed with corresponding Clang diffs introducing
+them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too
+old to support them, they shouldn't cause build failures or runtime test
+failures:
+
+  - __builtin_btf_type_id() ([0], [1], [2]);
+  - __builtin_preserve_type_info(), __builtin_preserve_enum_value() ([3], [4]).
+
+  [0] https://reviews.llvm.org/D74572
+  [1] https://reviews.llvm.org/D74668
+  [2] https://reviews.llvm.org/D85174
+  [3] https://reviews.llvm.org/D83878
+  [4] https://reviews.llvm.org/D83242

tools/testing/selftests/bpf/progs/test_core_reloc_type_id.c

···
 {
 	/* We use __builtin_btf_type_id() in this tests, but up until the time
 	 * __builtin_preserve_type_info() was added it contained a bug that
-	 * would make this test fail. The bug was fixed with addition of
+	 * would make this test fail. The bug was fixed ([0]) with addition of
 	 * __builtin_preserve_type_info(), though, so that's what we are using
 	 * to detect whether this test has to be executed, however strange
 	 * that might look like.
+	 *
+	 *   [0] https://reviews.llvm.org/D85174
 	 */
 #if __has_builtin(__builtin_preserve_type_info)
 	struct core_reloc_type_id_output *out = (void *)&data.out;