Minor cleanup of aidl.cpp

1) Move some helper functions to parse_helpers.cpp
2) Refactor some helper functions to take advantage of C++ built ins
3) Add unittest coverage of refactored logic
4) clang-format -i -style=Google parse_helpers.cpp
5) Move remaining helper functions of aidl.cpp into anonymous namespace

Change-Id: I3a2cf7be113ac7f71a1a3502b12a8ed910509546
diff --git a/Type.cpp b/Type.cpp
index 3ce93fe..31a8926 100644
--- a/Type.cpp
+++ b/Type.cpp
@@ -181,6 +181,19 @@
 {
 }
 
+string
+Type::HumanReadableKind() const {
+    switch (Kind())
+    {
+        case INTERFACE:
+            return "an interface";
+        case USERDATA:
+            return "a user data";
+        default:
+            return "ERROR";
+    }
+}
+
 bool
 Type::CanBeArray() const
 {