[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

llvm-svn: 374962