RequirementMachine: Take concrete substitutions into account when checking completion depth limit

We didn't look at the length of terms appearing in concrete substitutions,
so runaway recursion there was only caught by the completion step limit
which takes much longer.

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -72,6 +72,19 @@ bool Rule::isProtocolRefinementRule() const {
           LHS[0] != LHS[1]);
 }
 
+/// Returns the length of the left hand side.
+unsigned Rule::getDepth() const {
+  auto result = LHS.size();
+
+  if (LHS.back().isSuperclassOrConcreteType()) {
+    for (auto substitution : LHS.back().getSubstitutions()) {
+      result = std::max(result, substitution.size());
+    }
+  }
+
+  return result;
+}
+
 /// Linear order on rules; compares LHS followed by RHS.
 int Rule::compare(const Rule &other, RewriteContext &ctx) const {
   int compare = LHS.compare(other.LHS, ctx);

lib/AST/RequirementMachine/RewriteSystem.h

@@ -116,10 +116,7 @@ class Rule final {
     Redundant = true;
   }
 
-  /// Returns the length of the left hand side.
-  unsigned getDepth() const {
-    return LHS.size();
-  }
+  unsigned getDepth() const;
 
   int compare(const Rule &other, RewriteContext &ctx) const;