Add a new core type called Swizzle View that is used as the type of
swizzle expressions in the resolver. This change is based on the draft
spec in: gpuweb/gpuweb#5268

Currently, there should be no functional change, it simply changes the
sem node types, which are then lowered in ProgramToIR to the expected
vector types. However, this change supports a future implementation of
swizzle assignment, where it will be necessary for swizzled objects to
be treated as memory views rather than concrete values.

Bug: 473857629
Change-Id: I34342f921e6906b599f840c10dbb455eed37cb94
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/285675
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@google.com>
Reviewed-by: James Price <jrprice@google.com>

Add language extension 'swizzle_assignment'

Example:

   var v:vec4u;
   v.yz = vec2(1,2);

Now includes fixes on the order of evaluation.

Issue: gpuweb#737

…ions table

  We need to splite MVE memory-view expression from SVE
  swizzle-view-expression

When using a single-letter swizzle name on a pointer or reference,
the result is a reference to a component. This can be used
anywhere a reference-to-component can be used!

For reference, it's still twice.

For swizzle view, it's three times.

h/t @sudonatalie

split the tables between the case where the inner expression
is a vector vs. a memory view of a vector.

…zzle View

The result is always a reference, not just a memory view.