Backport boringssl patches to fix clang fall through warnings.
https://boringssl-review.googlesource.com/c/boringssl/+/37244
https://boringssl-review.googlesource.com/c/boringssl/+/37247

Author: Manoj Gupta <manojgupta@google.com>
Date:   Mon Aug 26 09:13:58 2019 -0700

    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

diff --git a/crypto/internal.h b/crypto/internal.h
index 52799e899..92f5313f1 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -187,6 +187,12 @@ typedef __uint128_t uint128_t;
 #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
diff --git a/crypto/asn1/tasn_enc.c b/crypto/asn1/tasn_enc.c
index d89ec8a73..3722a5191 100644
--- a/crypto/asn1/tasn_enc.c
+++ b/crypto/asn1/tasn_enc.c
@@ -192,7 +192,7 @@ int ASN1_item_ex_i2d(ASN1_VALUE **pval, unsigned char **out,
         /* Use indefinite length constructed if requested */
         if (aclass & ASN1_TFLG_NDEF)
             ndef = 2;
-        /* fall through */
+        OPENSSL_FALLTHROUGH;
 
     case ASN1_ITYPE_SEQUENCE:
         i = asn1_enc_restore(&seqcontlen, out, pval, it);
diff --git a/crypto/fipsmodule/cipher/cipher.c b/crypto/fipsmodule/cipher/cipher.c
index 39e038beb..d0682ab37 100644
--- a/crypto/fipsmodule/cipher/cipher.c
+++ b/crypto/fipsmodule/cipher/cipher.c
@@ -191,7 +191,7 @@ int EVP_CipherInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *cipher,
 
       case EVP_CIPH_CFB_MODE:
         ctx->num = 0;
-        // fall-through
+        OPENSSL_FALLTHROUGH;
 
       case EVP_CIPH_CBC_MODE:
         assert(EVP_CIPHER_CTX_iv_length(ctx) <= sizeof(ctx->iv));