Refactor libTooling to reduce required dependencies.

This moves classes for storing and applying replacements to separate
files. These classes specifically are used by clang-format which doesn't
have any other dependencies on clangAST. Thereby, the size of
clang-format's binary can be cut roughly in half and its build time sped
up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220867 91177308-0d34-0410-b5e6-96231b3b80d8