Add a test case for skipping synthesized members

Author: cmcgee1024

test/SymbolGraph/Symbols/SkipsSynthesizedMembers.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -module-name SkipsSynthesizedMembers -emit-module -emit-module-path %t/
+// RUN: %target-swift-symbolgraph-extract -module-name SkipsSynthesizedMembers -I %t -pretty-print -skip-synthesized-members -output-dir %t
+// RUN: %FileCheck %s --input-file %t/SkipsSynthesizedMembers.symbols.json
+
+// CHECK-NOT: ::SYNTHESIZED
+
+public struct ShouldAppear: Hashable {
+    public let foo: Int
+}