Add Fallthru support for clang 10.

clang has gained supoprted for __attribute__ ((fallthrough))
in https://reviews.llvm.org/rL369414.
Detect the support in clang and enable it as OPENSSL_FALLTHROUGH.
This is needed to fix ToT clang builds.

Bug: chromium:997709
Test: CQ
Change-Id: Iefa17687f6b5e8c95f359f167e9049d9a69c5302
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37244
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index 76a317a..3c682e8 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -187,6 +187,12 @@
 #define OPENSSL_FALLTHROUGH [[gnu::fallthrough]]
 #elif defined(__GNUC__) && __GNUC__ >= 7 // gcc 7
 #define OPENSSL_FALLTHROUGH __attribute__ ((fallthrough))
+#elif defined(__clang__)
+#if __has_attribute(fallthrough)
+#define OPENSSL_FALLTHROUGH __attribute__ ((fallthrough))
+#else // clang versions that do not support fallthrough.
+#define OPENSSL_FALLTHROUGH
+#endif
 #else // C++11 on gcc 6, and all other cases
 #define OPENSSL_FALLTHROUGH
 #endif