We aim to eventually make the entire X509 structure opaque, but let's
start small.
Update-Note: I believe this is now safe to do. If there are compile
failures, switch to X509_get0_notBefore, X509_getm_notBefore, and
X509_set1_notBefore, or revert this if I'm wrong and too many callers
still need updating.
Change-Id: I6e9d91630a10ac777e13ebcdeb543b3cbeea6383
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45965
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>