[SourceLocation] Extract and rework line tables

* `computeLines` now produces newly introducded 'SourceLineTable'.
* `SourceLineTable` is factored out line tables and `#sourceLocation`
  directive info, extracted from `SourceLocationConverter`.
* `forEachLineLength` is now `forEachEndOfLine` which emits the absolute
  position of each end-of-line, instead of the line length, because the
  caller can easily derive the start-of-line position (i.e. the last
  end-of-line).
* `#sourceLocation` info are now handled differently. Specifically, while
  traversing the syntax tree, it only collects the raw syntax, then
  post-process those info to produce the newly introduced `VirtualFile`.
  `VirtualFile` is a type that have enough infomation for
  `swift::SourceMamanger` in the compiler. Post-processing is needed
  because we want the start position of the _next_ line of the end of
  `#sourceLocation` directive, which might still not available when the
  first pass sees the directive.
* In the future, the compiler shoud only use `SourceLineTable` instead
  of `SourceLocationConverter` as `swift::SourceManager` already know the
  file name and the whole source buffer.

Author: rintaro