nested types: restructure GenerateCpp()

Each output(.h/.cpp) needs to emit nested types recursively. For
example, a header might contain decls of interface/parcelable/etc.

This change prepares upcoming changes for C++ nested types.

Bug: 201376182
Test: aidl_unittests, tests/golden_test.sh check
Change-Id: I7ae347e8ba3eb9661d7d36421647f1d299068f38
1 file changed