[CS] Avoid bailing on result builder return for completion

Make sure we continue to solve the body normally rather than bailing
early.

Author: hamishknight

lib/Sema/BuilderTransform.cpp

@@ -1132,8 +1132,9 @@ ConstraintSystem::matchResultBuilder(AnyFunctionRef fn, Type builderType,
   if (fn.bodyHasExplicitReturnStmt()) {
     // Diagnostic mode means that solver couldn't reach any viable
     // solution, so let's diagnose presence of a `return` statement
-    // in the closure body.
-    if (shouldAttemptFixes()) {
+    // in the closure body. Avoid doing this for completion since we need to
+    // continue solving the body.
+    if (shouldAttemptFixes() && !isForCodeCompletion()) {
       if (recordFix(IgnoreResultBuilderWithReturnStmts::create(
               *this, builderType,
               getConstraintLocator(fn.getAbstractClosureExpr()))))

test/IDE/complete_in_result_builder.swift

@@ -16,7 +16,10 @@ struct TupleBuilder<T> {
   }
 }
 
-func buildStringTuple<Result>(@TupleBuilder<String> x: () -> Result) {}
+@discardableResult
+func buildStringTuple<Result>(@TupleBuilder<String> x: () -> Result) -> Result {
+  x()
+}
 
 enum StringFactory {
   static func makeString(x: String) -> String { return x }
@@ -98,6 +101,21 @@ struct FooStruct {
   func intGen() -> Int { return 1 }
 }
 
+func testReturn() {
+  // Make sure we can still complete even with return.
+  buildStringTuple {
+    StringFactory.#^COMPLETE_STATIC_MEMBER_WITH_RETURN^#
+    // COMPLETE_STATIC_MEMBER_WITH_RETURN: Decl[StaticMethod]/CurrNominal:     makeString({#x: String#})[#String#];
+    return ""
+  }
+
+  buildStringTuple {
+    ""
+    return FooStruct()
+  }.#^COMPLETE_INSTANCE_MEMBER_ON_RETURN_BUILDER^#
+  // COMPLETE_INSTANCE_MEMBER_ON_RETURN_BUILDER: Decl[InstanceVar]/CurrNominal: instanceVar[#Int#]; name=instanceVar
+}
+
 func testPatternMatching() {
   @TupleBuilder<String> var x1 {
     let x = Letters.b