RequirementMachine: Only check local rules against maximum rule limit

Completion has a maximum rule limit since the Knuth-Bendix algorithm
is a semi-decision procedure that does not terminate on all inputs.

However, this means that generic signatures which import a large
number of complex protocols can hit the limit even if they are
convergent.

Since imported rules are essentially free, ignore them here, to
avoid having to increase the limit by hand.

Now the default limit is 4000 local rules per requirement machine;
it seems implausible that you would exceed this, but if anyone has
an example we can bump the limit again.

Author: slavapestov

lib/AST/RequirementMachine/KnuthBendix.cpp

@@ -391,7 +391,7 @@ RewriteSystem::computeConfluentCompletion(unsigned maxRuleCount,
 
     for (const auto &pair : resolvedCriticalPairs) {
       // Check if we've already done too much work.
-      if (Rules.size() > maxRuleCount)
+      if (getLocalRules().size() > maxRuleCount)
         return std::make_pair(CompletionResult::MaxRuleCount, Rules.size() - 1);
 
       if (!addRule(pair.LHS, pair.RHS, &pair.Path))

lib/AST/RequirementMachine/RequirementMachine.cpp

@@ -305,7 +305,7 @@ RequirementMachine::computeCompletion(RewriteSystem::ValidityPolicy policy) {
         }
       }
 
-      if (System.getRules().size() > MaxRuleCount) {
+      if (System.getLocalRules().size() > MaxRuleCount) {
         return std::make_pair(CompletionResult::MaxRuleCount,
                               System.getRules().size() - 1);
       }