Update string shape from synthetic to base (#549)

Author: Madrigal

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/util/ShapeUtil.java

@@ -27,7 +27,7 @@
 
 public final class ShapeUtil {
     public static final StringShape STRING_SHAPE = StringShape.builder()
-            .id("smithy.go.synthetic#String")
+            .id("smithy.api#String")
             .build();
 
     public static final IntegerShape INT_SHAPE = IntegerShape.builder()

codegen/smithy-go-codegen/src/test/java/software/amazon/smithy/go/codegen/GoJmespathExpressionGeneratorTest.java

@@ -492,4 +492,33 @@ public void testMultiSelect() {
                 v3 := []*string{v1,v2}
                 """));
     }
+
+    @Test
+    public void testMultiSelectFlatten() {
+        var expr = "objectList[*].[key][]";
+
+        var writer = testWriter();
+        var generator = new GoJmespathExpressionGenerator(testContext(), writer);
+        var actual = generator.generate(JmespathExpression.parse(expr), new GoJmespathExpressionGenerator.Variable(
+                TEST_MODEL.expectShape(ShapeId.from("smithy.go.test#Struct")),
+                "input"
+        ));
+        assertThat(actual.shape().toShapeId().toString(), Matchers.equalTo("smithy.go.synthetic#StringList"));
+        assertThat(actual.ident(), Matchers.equalTo("v5"));
+        assertThat(writer.toString(), Matchers.containsString("""
+                v1 := input.ObjectList
+                var v2 [][]string
+                for _, v := range v1 {
+                    v3 := v.Key
+                    v4 := []*string{v3}
+                    if v4 != nil {
+                        v2 = append(v2, *v4)
+                    }
+                }
+                var v5 []string
+                for _, v := range v2 {
+                    v5 = append(v5, v...)
+                }
+                """));
+    }
 }