Add clang-reorder-fields to clang-tools-extra

This diff adds v0 of clang-reorder-fields tool to clang/tools/extra.
The main idea behind this tool is to simplify and make less error-prone refactoring of large codebases when
someone needs to change the order fields of a struct/class (for example to remove excess padding).

Differential revision: https://reviews.llvm.org/D23279

llvm-svn: 280431
GitOrigin-RevId: ae4ff453a4a95140669dd0cab28ec7d7032e5449
13 files changed