SERVER-27244 Status usage compile-time facilities.

There are numerous places in the codebase where `mongo::Status` or
`mongo::StatusWith< T >` objects are returned and never checked.
Many of these are innocuous, but many of them are potentially
severe bugs.  This change introduces facilities to permit
compile-time warning of unchecked `Status` and `StatusWith` usage
on clang compilers.  It introduces an `ignore` function which is
useful to state that a specific "ignored status" case was
intentional.  It not presently an error, in clang builds, to
forget to check a `Status` -- this will come in a later commit.

This also introduces a `transitional_ignore` function, which allows
for easy continual auditing of the codebase for current "whitelisted"
unchecked-status instances.  All present "ignored status" cases
have been marked `transitional_ignore`.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/mongo/db/clientcursor.cpp b/src/mongo/db/clientcursor.cpp
index 889d9961607..4f169b2fa1f 100644
--- a/src/mongo/db/clientcursor.cpp
+++ b/src/mongo/db/clientcursor.cpp
@@ -140,7 +140,9 @@ void ClientCursor::updateSlaveLocation(OperationContext* opCtx) {
     if (!rid.isSet())
         return;
 
-    repl::getGlobalReplicationCoordinator()->setLastOptimeForSlave(rid, _slaveReadTill);
+    repl::getGlobalReplicationCoordinator()
+        ->setLastOptimeForSlave(rid, _slaveReadTill)
+        .transitional_ignore();
 }
 
 //