[CSFix] Introduce non-fatal "warning" fixes

The intention here is to be able to detect warnings earlier and
move some of the logic from CSApply and MiscDiagnostics to solver.

Warning fixes still lead to solution being applied to AST.

Author: xedin

lib/Sema/CSApply.cpp

@@ -7808,23 +7808,31 @@ bool ConstraintSystem::applySolutionFixes(Expr *E, const Solution &solution) {
     if (fixes == fixesPerExpr.end())
       return false;
 
-    bool diagnosed = false;
-    for (const auto *fix : fixes->second)
-      diagnosed |= fix->diagnose(E);
-    return diagnosed;
+    bool diagnosedError = false;
+    for (const auto *fix : fixes->second) {
+      auto diagnosed = fix->diagnose(E);
+
+      if (fix->isWarning()) {
+        assert(diagnosed && "warnings should always be diagnosed");
+        (void)diagnosed;
+      } else {
+        diagnosedError |= diagnosed;
+      }
+    }
+    return diagnosedError;
   };
 
-  bool diagnosed = false;
+  bool diagnosedError = false;
   E->forEachChildExpr([&](Expr *subExpr) -> Expr * {
     // Diagnose root expression at the end to
     // preserve ordering.
     if (subExpr != E)
-      diagnosed |= diagnoseExprFailures(subExpr);
+      diagnosedError |= diagnoseExprFailures(subExpr);
     return subExpr;
   });
 
-  diagnosed |= diagnoseExprFailures(E);
-  return diagnosed;
+  diagnosedError |= diagnoseExprFailures(E);
+  return diagnosedError;
 }
 
 /// Apply a given solution to the expression, producing a fully
@@ -7839,15 +7847,21 @@ Expr *ConstraintSystem::applySolution(Solution &solution, Expr *expr,
     if (shouldSuppressDiagnostics())
       return nullptr;
 
-    // If we can diagnose the problem with the fixits that we've pre-assumed,
-    // do so now.
-    if (applySolutionFixes(expr, solution))
-      return nullptr;
+    bool diagnosedErrorsViaFixes = applySolutionFixes(expr, solution);
+    // If all of the available fixes would result in a warning,
+    // we can go ahead and apply this solution to AST.
+    if (!llvm::all_of(solution.Fixes, [](const ConstraintFix *fix) {
+          return fix->isWarning();
+        })) {
+      // If we already diagnosed any errors via fixes, that's it.
+      if (diagnosedErrorsViaFixes)
+        return nullptr;
 
-    // If we didn't manage to diagnose anything well, so fall back to
-    // diagnosing mining the system to construct a reasonable error message.
-    diagnoseFailureForExpr(expr);
-    return nullptr;
+      // If we didn't manage to diagnose anything well, so fall back to
+      // diagnosing mining the system to construct a reasonable error message.
+      diagnoseFailureForExpr(expr);
+      return nullptr;
+    }
   }
 
   // Mark any normal conformances used in this solution as "used".

lib/Sema/CSFix.h

@@ -111,14 +111,21 @@ class ConstraintFix {
   FixKind Kind;
   ConstraintLocator *Locator;
 
+  /// Determines whether this fix is simplify a warning which doesn't
+  /// require immediate source changes.
+  bool IsWarning;
+
 public:
-  ConstraintFix(ConstraintSystem &cs, FixKind kind, ConstraintLocator *locator)
-      : CS(cs), Kind(kind), Locator(locator) {}
+  ConstraintFix(ConstraintSystem &cs, FixKind kind, ConstraintLocator *locator,
+                bool warning = false)
+      : CS(cs), Kind(kind), Locator(locator), IsWarning(warning) {}
 
   virtual ~ConstraintFix();
 
   FixKind getKind() const { return Kind; }
 
+  bool isWarning() const { return IsWarning; }
+
   virtual std::string getName() const = 0;
 
   /// Diagnose a failure associated with this fix given