refactor: make "Autofill" explicit

Extracted "Autofill" (setting backing-type for enums) into a separate
method which is called by AidlTypenames::Autofill().

Now the processing steps are as follows: (see diagnostics_unittest.cpp)
- Parser::Parse(): load a doc into AST
- parser.Resolve(): type/reference resolution(link references to targets)
- typenames.Autofill(): fill or fix AST to be valid
- CheckValid: is AST valid?
- .. further processing ..

Bug: none
Test: m aidl_unittests
Change-Id: I70117aa6df8301b8665194f4a0466df1f271aa67
6 files changed