cpp11-migrate: New mechanism for overriding file contents

Next step toward supporting migrating headers. Instead of using ClangTool's
ability to override all files at once, use a custom FrontendAction and override
only the source (and eventually headers) the action is about to parse.

Use of newFrontendActionFactory() is replaced with a new factory maker provided
by Transform.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@183855 91177308-0d34-0410-b5e6-96231b3b80d8
cpp11-migrate: Decluttering Transform.h

Moving RewriteContainer's constructor impl to Transform.cpp to reduce clutter.
More things will soon be added to this header.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@183856 91177308-0d34-0410-b5e6-96231b3b80d8