[CSSolver] Don't re-run solver to diagnose ambiguities

If ambiguity has been detected during normal solving,
let's diagnose that right away instead of re-running
the solver in diagnostic mode because it would produce
exactly the same set of solutions (all solutions with
fixes added in diagnostic mode would be filtered out
because they'd have worse scores).

Author: xedin

lib/Sema/CSSolver.cpp

@@ -1475,6 +1475,23 @@ ConstraintSystem::solve(SyntacticElementTarget &target,
         return std::move(result);
       }
 
+      auto solutionsRef = std::move(solution).takeAmbiguousSolutions();
+      SmallVector<Solution, 4> ambiguity(
+          std::make_move_iterator(solutionsRef.begin()),
+          std::make_move_iterator(solutionsRef.end()));
+
+      {
+        SolverState state(*this, FreeTypeVariableBinding::Disallow);
+
+        // Constraint generator is allowed to introduce fixes to the
+        // contraint system.
+        if (diagnoseAmbiguityWithFixes(ambiguity) ||
+            diagnoseAmbiguity(ambiguity)) {
+          solution.markAsDiagnosed();
+          return None;
+        }
+      }
+
       LLVM_FALLTHROUGH;
     }
 
@@ -1569,7 +1586,7 @@ bool ConstraintSystem::solve(SmallVectorImpl<Solution> &solutions,
   // Filter deduced solutions, try to figure out if there is
   // a single best solution to use, if not explicitly disabled
   // by constraint system options.
-  filterSolutions(solutions);
+  filterSolutions(solutions, /*minimize=*/true);
 
   // We fail if there is no solution or the expression was too complex.
   return solutions.empty() || isTooComplex(solutions);