Restore CHECKED_CAST.

Although this macro is not public API and is unused in BoringSSL,
wpa_supplicant uses it to define its own stacks. Remove this once
wpa_supplicant has been fixed.

Change-Id: I1f85e06efe4057b6490bf93bf4dea773dcb491c5
Reviewed-on: https://boringssl-review.googlesource.com/c/32764
Reviewed-by: Robert Sloan <varomodt@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/type_check.h

diff --git a/include/openssl/type_check.h b/include/openssl/type_check.h
index 909f931..90d81f7 100644
--- a/include/openssl/type_check.h
+++ b/include/openssl/type_check.h
@@ -79,6 +79,13 @@
 #define OPENSSL_COMPILE_ASSERT(cond, msg) _Static_assert(cond, #msg)
 #endif
 
+// CHECKED_CAST casts |p| from type |from| to type |to|.
+//
+// TODO(davidben): Although this macro is not public API and is unused in
+// BoringSSL, wpa_supplicant uses it to define its own stacks. Remove this once
+// wpa_supplicant has been fixed.
+#define CHECKED_CAST(to, from, p) ((to) (1 ? (p) : (from)0))
+
 
 #if defined(__cplusplus)
 }  // extern C