Replace SkSL::Type constructors with named factory functions.

This makes it much easier to understand what sorts of types we are
creating.

This has some minor repercussions for the SPIR-V code generator, which
actually created temporary Types on the stack occasionally, but these
were simple to fix.

Change-Id: I1ca43cdef0445d2b9789a435221dce50b03d954a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343517
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
9 files changed