Sema: Contextualize closures for function builder bodies also

The ContextualizeClosures walker re-parents and assigns discriminators to
autoclosures. We weren't doing this walk for function builder bodies,
which meant that proper invariants were not being established for the
implicit autoclosures synthesized for partially applied method references.

This is a recent regression from my change to build curry thunks for
unapplied method references in Sema.

Fixes <rdar://problem/61039516>.

Author: slavapestov

lib/Sema/TypeCheckStmt.cpp

@@ -1911,6 +1911,8 @@ TypeCheckFunctionBodyUntilRequest::evaluate(Evaluator &evaluator,
 
         body = *optBody;
         alreadyTypeChecked = true;
+
+        body->walk(ContextualizeClosures(AFD));
       }
     } else if (func->hasSingleExpressionBody() &&
                func->getResultInterfaceType()->isVoid()) {

test/SILGen/function_builder_curry_thunks.swift

@@ -0,0 +1,33 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+@_functionBuilder
+struct Builder {
+  static func buildBlock<T1>(_ t1: T1) -> (T1) {
+    return (t1)
+  }
+}
+
+struct Handler {
+  var firstHandler: () -> ()
+  var secondHandler: () -> ()
+}
+
+// We were neglecting to assign discriminators and re-parent
+// autoclosures, which would manifest as curried method references
+// producing a bogus diagnostic about captures from inside a
+// nested type.
+class Outer {
+  struct Inner {
+    @Builder
+    var build: Handler {
+      Handler(firstHandler: self.handler, secondHandler: self.handler)
+    }
+
+    private func handler() {}
+  }
+}
+
+// CHECK-LABEL: sil private [ossa] @$s29function_builder_curry_thunks5OuterC5InnerV5buildAA7HandlerVvgyycAEcfu_ : $@convention(thin) (Outer.Inner) -> @owned @callee_guaranteed () -> ()
+// CHECK-LABEL: sil private [ossa] @$s29function_builder_curry_thunks5OuterC5InnerV5buildAA7HandlerVvgyycAEcfu_yycfu0_ : $@convention(thin) (Outer.Inner) -> ()
+// CHECK-LABEL: sil private [ossa] @$s29function_builder_curry_thunks5OuterC5InnerV7handler33_DC254A3F89F9C7E65D25434E199F17A4LLyyF : $@convention(method) (Outer.Inner) -> ()
+// CHECK-LABEL: sil private [ossa] @$s29function_builder_curry_thunks5OuterC5InnerV5buildAA7HandlerVvgyycAEcfu1_ : $@convention(thin) (Outer.Inner) -> @owned @callee_guaranteed () -> ()