Fixes failed compile with [-Werror=implicit-fallthrough=], which is
default on gcc-7.x on distributions like fedora.

Enabling no implicit fallthrough for more than just clang as well to
catch this going forward.

Change-Id: I6cd880dac70ec126bd7812e2d9e5ff804d32cadd
Signed-off-by: Vincent Batts <vbatts@redhat.com>
Reviewed-on: https://boringssl-review.googlesource.com/20564
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/{asn1,x509,x509v3,pem} were skipped as they are still OpenSSL
style.

Change-Id: I3cd9a60e1cb483a981aca325041f3fbce294247c
Reviewed-on: https://boringssl-review.googlesource.com/19504
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

Change-Id: I167b7045c537d95294d387936f3d7bad530e1c6f
Reviewed-on: https://boringssl-review.googlesource.com/15844
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>