Add OPENSSL_FALLTHROUGH to a few files.

This is a followup of
https://boringssl-review.googlesource.com/c/boringssl/+/37244

There are a few files that needs the OPENSSL_FALLTHROUGH
annotation so that they compile with clang.

Bug: chromium:997709
Test: CQ
Change-Id: I05f9f85fdb39fbcb8c1193a7b0c335287022719d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37247
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

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);

crypto/fipsmodule/cipher/cipher.c

@@ -192,7 +192,7 @@ 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));