Made SkSL type aliases into first-class objects

Previously, type aliases ('vec2') were just an additional name which
could be used to refer to a type ('float2'). This was simple and worked,
except that error messages would be wrong - any type-related error
message would refer to the type as 'float2' rather than the 'vec2' that
the user actually typed.

This CL adds an AliasType class so that we can track which name was
used to refer to an aliased type and report messages using the correct
type name.

Bug: skia:12737

Change-Id: I40e234239ab47557033e0695e4fbbd5f01da354e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/490256
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed