[libTooling] Put all Transformer declarations in a single namespace.

[libTooling] Put all Transformer declarations in a single namespace.

Summary:
This revision introduces a new namespace, `clang::transformer`, to hold
the declarations for the Transformer library.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68876

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374962 91177308-0d34-0410-b5e6-96231b3b80d8