[NFC] LifetimeCompletion: Improve lambda name.

This lambda has to do with availability or the lack thereof but
unreachable instructions are of interest in this context so using the
wrong name confuses things unnecessarily.

Author: nate-chandler

lib/SIL/Utils/OSSALifetimeCompletion.cpp

@@ -291,16 +291,16 @@ void VisitUnreachableLifetimeEnds::visitAvailabilityBoundary(
     if (!available) {
       continue;
     }
-    auto hasUnreachableSuccessor = [&]() {
+    auto hasUnavailableSuccessor = [&]() {
       // Use a lambda to avoid checking if possible.
       return llvm::any_of(block->getSuccessorBlocks(), [&result](auto *block) {
         return result.getState(block) == State::Unavailable;
       });
     };
-    if (!block->succ_empty() && !hasUnreachableSuccessor()) {
+    if (!block->succ_empty() && !hasUnavailableSuccessor()) {
       continue;
     }
-    assert(hasUnreachableSuccessor() ||
+    assert(hasUnavailableSuccessor() ||
            isa<UnreachableInst>(block->getTerminator()));
     visit(block->getTerminator());
   }