[ClangTidy] Add UsingInserter and NamespaceAliaser

Summary: This adds helper classes to add using declaractions and namespace aliases to function bodies. These help making function calls to deeply nested functions concise (e.g. when calling helpers in a refactoring)

Patch by Julian Bangert!

Reviewers: alexfh, hokein

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D24997

llvm-svn: 283981