[ConstraintSystem] Add isStaticallyDerivedMetatype to Solution

This is useful for a new style code completion which doesn't apply
solutions back to neither constraint system nor AST.

Author: xedin

lib/Sema/CSApply.cpp

@@ -176,12 +176,46 @@ bool ConstraintSystem::isTypeReference(Expr *E) {
       });
 }
 
+bool Solution::isTypeReference(Expr *E) const {
+  return E->isTypeReference(
+      [&](Expr *expr) -> Type { return simplifyType(getType(expr)); },
+      [&](Expr *expr) -> Decl * {
+        ConstraintLocator *locator = nullptr;
+        if (auto *UDE = dyn_cast<UnresolvedDotExpr>(E)) {
+          locator = getConstraintLocator(UDE, {ConstraintLocator::Member});
+        }
+
+        if (auto *UME = dyn_cast<UnresolvedMemberExpr>(E)) {
+          locator =
+              getConstraintLocator(UME, {ConstraintLocator::UnresolvedMember});
+        }
+
+        if (isa<OverloadSetRefExpr>(E))
+          locator = getConstraintLocator(const_cast<Expr *>(E));
+
+        if (locator) {
+          if (auto selectedOverload = getOverloadChoiceIfAvailable(locator)) {
+            const auto &choice = selectedOverload->choice;
+            return choice.getDeclOrNull();
+          }
+        }
+
+        return nullptr;
+      });
+}
+
 bool ConstraintSystem::isStaticallyDerivedMetatype(Expr *E) {
   return E->isStaticallyDerivedMetatype(
       [&](Expr *E) -> Type { return simplifyType(getType(E)); },
       [&](Expr *E) -> bool { return isTypeReference(E); });
 }
 
+bool Solution::isStaticallyDerivedMetatype(Expr *E) const {
+  return E->isStaticallyDerivedMetatype(
+      [&](Expr *E) -> Type { return simplifyType(getType(E)); },
+      [&](Expr *E) -> bool { return isTypeReference(E); });
+}
+
 Type ConstraintSystem::getInstanceType(TypeExpr *E) {
   if (!hasType(E))
     return Type();

lib/Sema/ConstraintSystem.h

@@ -1210,6 +1210,16 @@ class Solution {
         : nullptr;
   }
 
+  /// This method implements functionality of `Expr::isTypeReference`
+  /// with data provided by a given solution.
+  bool isTypeReference(Expr *E) const;
+
+  /// Call Expr::isIsStaticallyDerivedMetatype on the given
+  /// expression, using a custom accessor for the type on the
+  /// expression that reads the type from the Solution
+  /// expression type map.
+  bool isStaticallyDerivedMetatype(Expr *E) const;
+
   SWIFT_DEBUG_DUMP;
 
   /// Dump this solution.