This reverts commit f2d000328f.
Reason for revert: breaking google3 roll
Original change's description:
> Eliminate DSLPossibleExpression.
>
> The Possible(Expression|Statement) classes were added at
> http://review.skia.org/375069. These classes were responsible for
> capturing `__builtin_FILE()` and `__builtin_LINE()` when an
> expression or statement was added to a hand-authored DSL program. This
> allowed errors to be reported on the C++ file/line where they were
> encountered. This was a good feature to have, when the plan was to
> author the majority of SkSL code via DSL.
>
> Later, IRNode positions were converted from an integer line number to
> SkSL Positions at http://review.skia.org/518137. This gave us range
> tracking, but at a high memory cost (16 bytes per IRNode, versus four
> bytes when we tracked line numbers only).
>
> Positions were reduced to 8 bytes at http://review.skia.org/521005 by
> removing the filename, which was only used for hand-authored DSL. (The
> size was pared all the way back to 4 bytes at
> http://review.skia.org/533699 by packing the data more efficiently.)
>
> __builtin_FILE/LINE capturing was removed entirely at
> http://review.skia.org/528366; the filename was discarded anyway and
> the line number didn't have a range and wasn't very meaningful without
> a filename. Also, it didn't matter very much since we no longer intended
> to hand-craft our programs in DSL.
>
> At this stage, DSLPossibleExpression stopped adding value and simply
> served to move Expressions around.
>
> Change-Id: I29ac33e08dcc1804cc0619c1e8856ba28ebcc51d
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/542145
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>
Change-Id: I33badbdcce8760200246bf50e4932d42721ea952
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/543078
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Owners-Override: Kevin Lubick <kjlubick@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
193c16380f