Implements the features requested in PR14679.

- adds '-h' as an alias for '-help'.
- adds an optional <file> argument
- adds an option -i to in-place edit the given file
- adds a help text to explain what the tool does

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@171858 91177308-0d34-0410-b5e6-96231b3b80d8