[BuildTransform] Fix walkExplicitReturnStmts to walk implicit statements

Some statements introduce implicit braces and other things,
`walkExplicitReturnStmts` cannot ignore that while trying
to find explicit returns.

Resolves: rdar://139235128

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -1260,12 +1260,8 @@ static bool walkExplicitReturnStmts(const BraceStmt *BS,
     }
 
     PreWalkResult<Stmt *> walkToStmtPre(Stmt *S) override {
-      if (S->isImplicit()) {
-        return Action::SkipNode(S);
-      }
-
       auto *returnStmt = dyn_cast<ReturnStmt>(S);
-      if (!returnStmt) {
+      if (!returnStmt || returnStmt->isImplicit()) {
         return Action::Continue(S);
       }
 

validation-test/Sema/type_checker_crashers_fixed/rdar139235128.swift

@@ -0,0 +1,30 @@
+// RUN: %target-typecheck-verify-swift
+
+@resultBuilder
+struct Builder {
+  static func buildExpression<T: BinaryInteger>(_: T) {}
+  static func buildBlock<T>(_: T) {}
+}
+
+enum E {
+  case a(Int)
+}
+
+protocol Test {
+  associatedtype V: BinaryInteger
+  @Builder var prop: V { get }
+}
+
+struct S : Test {
+  var flag: E
+
+  var prop: some BinaryInteger {
+    switch flag {
+    case .a:
+      test()
+      return 42
+    }
+  }
+
+  func test() {}
+}