minor SkSL changes to avoid compiler errors in Chromium

minor SkSL changes to avoid compiler errors in Chromium

* fixed a couple of spots where using { } instead of an explicit constructor call resulted in errors
* Type::Field had a deleted copy constructor and therefore was not working inside std::vector; had to remove const from fields and change fType from a reference to a pointer

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2300023002

Review-Url: https://codereview.chromium.org/2300023002