AST: Cleanup RecordLayoutBuilder

No functionality changed, just some cleanups:
- Switch some loops to range-based for.
- Name some iterators with a more creative name than "I".
- Reduce dependence on auto. Does RD->bases() give you a list of
  CXXBaseSpecifiers or CXXRecordDecls? It's more clear to just say which
  upfront.

llvm-svn: 213121