Silence gcc warning on TimeManager::GetSafeTimeUnlocked()
Silence gcc warning on TimeManager::GetSafeTimeUnlocked()

gcc is issuing this warning:
../../src/kudu/consensus/time_manager.cc: In member function ‘kudu::Timestamp kudu::consensus::TimeManager::GetSafeTimeUnlocked()’:
../../src/kudu/consensus/time_manager.cc:232:1: warning: control reaches end of non-void function [-Wreturn-type]

This patch silences it by using '__builtin_unreachable()'.

Change-Id: I7a87b483453115ae34615698017482a0364b5dca
Reviewed-on: http://gerrit.cloudera.org:8080/5381
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Tested-by: Kudu Jenkins

helifu/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu