[CSSolver] Shrinking should avoid solutions with implicit conversions

Just like generic overloads, `shrink` should always avoid any solutions
with implicit conversions.

Reducing disjunction domains becaused on solutions with implicit
conversions could have negative performance impact due to the
increase in total number of solutions that have to be examined
as a result.

Author: xedin

Diff for: lib/Sema/CSSolver.cpp

@@ -742,6 +742,14 @@ void ConstraintSystem::Candidate::applySolutions(
   llvm::SmallDenseMap<OverloadSetRefExpr *, llvm::SmallSetVector<ValueDecl *, 2>>
     domains;
   for (auto &solution : solutions) {
+    auto &score = solution.getFixedScore();
+
+    // Avoid any solutions with implicit value conversions
+    // because they might get reverted later when more context
+    // becomes available.
+    if (score.Data[SK_ImplicitValueConversion] > 0)
+      continue;
+
     for (auto choice : solution.overloadChoices) {
       // Some of the choices might not have locators.
       if (!choice.getFirst())