[SourceLocation] Extract and rework line tables

* `computeLines` now produces newly introducded 'SourceLineTable'.
* `SourceLineTable` is factored out line tables and source location
  directive info from `SourceLocationConverter`.
* In the future, the compiler shoud only use `SourceLineTable` instead
  of `SourceLocationConverter` because its SourceManager already know
  the file name, and the whole source buffer.
* `forEachLineLength` is now `forEachEndOfLine` which emits the
  absolution position of each end-of-lne, instead of the line length.
* `#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.

Author: rintaro