Replace OPENSSL_STATIC_ASSERT with static_assert.

The C11 change has survived for three months now. Let's start freely
using static_assert. In C files, we need to include <assert.h> because
it is a macro. In C++ files, it is a keyword and we can just use it. (In
MSVC C, it is actually also a keyword as in C++, but close enough.)

I moved one assert from ssl3.h to ssl_lib.cc. We haven't yet required
C11 in our public headers, just our internal files.

Change-Id: Ic59978be43b699f2c997858179a9691606784ea5
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/53665
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 4d3ad44..56bc5c2 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -166,6 +166,10 @@
 
 BSSL_NAMESPACE_BEGIN
 
+static_assert(SSL3_RT_MAX_ENCRYPTED_OVERHEAD >=
+                  SSL3_RT_SEND_MAX_ENCRYPTED_OVERHEAD,
+              "max overheads are inconsistent");
+
 // |SSL_R_UNKNOWN_PROTOCOL| is no longer emitted, but continue to define it
 // to avoid downstream churn.
 OPENSSL_DECLARE_ERROR_REASON(SSL, UNKNOWN_PROTOCOL)