[clangd] Expose offset <-> LSP position functions, and fix bugs

Summary:
- Moved these functions to SourceCode.h
- added unit tests
- fix off by one in positionToOffset: Offset - 1 in final calculation was wrong
- fixed formatOnType which had an equal and opposite off-by-one
- positionToOffset and offsetToPosition both consistently clamp to beginning/end
  of file when input is out of range
- gave variables more descriptive names
- removed windows line ending fixmes where there is nothing to fix
- elaborated on UTF-8 fixmes

This will conflict with Eric's D41281, but in a pretty easy-to-resolve way.

Reviewers: ioeric

Subscribers: klimek, mgorny, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41351

llvm-svn: 321073