[CSGen] Unify handling of single- and multi-statement closures

Both multi- and single-statement closures now behave the same way
when it comes to constraint generation, because the body is opened
only once contextual type becomes available or it's impossible
to find one e.g. `_ = { 1 }`.

Author: xedin

lib/Sema/CSGen.cpp

@@ -3662,20 +3662,16 @@ namespace {
         }
       }
 
-      // For closures containing only a single expression, the body participates
-      // in type checking.
+      // Both multi- and single-statement closures now behave the same way
+      // when it comes to constraint generation.
       if (auto closure = dyn_cast<ClosureExpr>(expr)) {
         auto &CS = CG.getConstraintSystem();
-        if (closure->hasSingleExpressionBody()) {
-          auto closureType = CG.visitClosureExpr(closure);
-          if (!closureType)
-            return {false, nullptr};
+        auto closureType = CG.visitClosureExpr(closure);
+        if (!closureType)
+          return {false, nullptr};
 
-          CS.setType(expr, closureType);
-          return {false, expr};
-        }
-
-        return { true, expr };
+        CS.setType(expr, closureType);
+        return {false, expr};
       }
 
       // Don't visit CoerceExpr with an empty sub expression. They may occur
@@ -3736,12 +3732,6 @@ namespace {
         }
       }
 
-      if (auto closure = dyn_cast<ClosureExpr>(expr)) {
-        if (closure->hasSingleExpressionBody()) {
-          return expr;
-        }
-      }
-
       if (auto type = CG.visit(expr)) {
         auto simplifiedType = CS.simplifyType(type);