[clang-rename] cleanup `auto` usages

As Alexander pointed out, LLVM Coding Standards are more conservative about
using auto, i.e. it should be used in the following situations:

* When the type is obvious, i.e. explicitly mentioned in the same expression.
For example `if (const clang::FieldDecl *FieldDecl = Initializer->getMember())`.
* When the type is totally non-obvious and one iterates over something. For
example
`for (const auto &CurrDecl : Context.getTranslationUnitDecl()->decls())`.

Otherwise the type should be explicitly stated.

Reviewers: alexfh

Differential Revision: https://reviews.llvm.org/D23397

llvm-svn: 278760