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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213121 91177308-0d34-0410-b5e6-96231b3b80d8