SkSL DSL now reports the C++ line numbers of errors

This is done a best-effort basis. Positions will only be reported in
Clang and GCC, and may not be reported in every possible situation.

Change-Id: I7efb15781546b95e667fee34e488366174bb5f36
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374600
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed