Rename GuaranteedARCOpts to MandatoryARCOpts.

This bleeds into the implementation where "guaranteed" is used
everywhere to talk about optimization of guaranteed values. We need to
use mandatory to indicate we're talking about the pass pipeline.

lib/SILOptimizer/SemanticARC/LoadCopyToLoadBorrowOpt.cpp

@@ -321,7 +321,7 @@ static bool isWrittenTo(Context &ctx, LoadInst *load,
 bool SemanticARCOptVisitor::visitLoadInst(LoadInst *li) {
   // This optimization can use more complex analysis. We should do some
   // experiments before enabling this by default as a guaranteed optimization.
-  if (ctx.onlyGuaranteedOpts)
+  if (ctx.onlyMandatoryOpts)
     return false;
 
   // If we are not supposed to perform this transform, bail.