tint/msl: Preserve trailing vec3 padding

In order to preserve padding properly for MSL, we need to use its
packed_vec type for all vec3 types in storage buffers, not just struct
members. This commit includes a complete rewrite of the PackedVec3
transform to achieve this. The key details are:

* An internal `__packed_vec3<>` type was added, which corresponds to a
  `type::Vector` with an additional flag to indicate that it will be
  emitted as packed vector.

* The `PackedVec3` transform replaces all vec3 types used in
  host-shareable address spaces with the internal `__packed_vec3`
  type. This includes vec3 types that appear as the store type of a
  pointer.

* When used as an array element, these `__packed_vec3` types are
  wrapped in a struct that contains a single `__packed_vec3`
  member. This allows us to add an `@align()` attribute that ensures
  that `array<vec3<T>>` still has the correct array element stride.

* When the `vec3<T>` appears as a struct member in the input program,
  we apply the `@align()` to that member to ensure that we do not
  change its offset.

* Matrix types with three rows that are used in memory are replaced
  with an array of columns, where each column uses a `__packed_vec3`
  inside an aligned wrapper structure as above.

* Accesses to host-shareable memory that involve any of these types
  invoke a "pack" or "unpack" helper function to convert them to the
  equivalent type that uses `__packed_vec3` or a regular `vec3` as
  required.

* The `chromium_internal_relaxed_uniform_layout` extension is used to
  avoid issues where modifying a type in the uniform address space
  triggers stricter layout validation rules.

Bug: tint:1571
Fixed: tint:1837
Change-Id: Idaf2da2f5bcb2be00c85ec657edfb614186476bb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/121200
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/buffer/uniform/std140/array/mat4x2_f16/to_builtin.wgsl.expected.msl

@@ -16,8 +16,8 @@ struct tint_array {
 
 kernel void f(const constant tint_array<half4x2, 4>* tint_symbol [[buffer(0)]]) {
   half2x4 const t = transpose((*(tint_symbol))[2]);
-  half const l = length(half2((*(tint_symbol))[0][1]).yx);
-  half const a = fabs(half2((*(tint_symbol))[0][1]).yx[0]);
+  half const l = length((*(tint_symbol))[0][1].yx);
+  half const a = fabs((*(tint_symbol))[0][1].yx[0]);
   return;
 }
 

test/tint/buffer/uniform/std140/array/mat4x2_f16/to_fn.wgsl.expected.msl

@@ -29,8 +29,8 @@ void d(half f_1) {
 kernel void f(const constant tint_array<half4x2, 4>* tint_symbol [[buffer(0)]]) {
   a(*(tint_symbol));
   b((*(tint_symbol))[1]);
-  c(half2((*(tint_symbol))[1][0]).yx);
-  d(half2((*(tint_symbol))[1][0]).yx[0]);
+  c((*(tint_symbol))[1][0].yx);
+  d((*(tint_symbol))[1][0].yx[0]);
   return;
 }
 

test/tint/buffer/uniform/std140/array/mat4x2_f16/to_private.wgsl.expected.msl

@@ -18,7 +18,7 @@ kernel void f(const constant tint_array<half4x2, 4>* tint_symbol_1 [[buffer(0)]]
   thread tint_array<half4x2, 4> tint_symbol = {};
   tint_symbol = *(tint_symbol_1);
   tint_symbol[1] = (*(tint_symbol_1))[2];
-  tint_symbol[1][0] = half2((*(tint_symbol_1))[0][1]).yx;
+  tint_symbol[1][0] = (*(tint_symbol_1))[0][1].yx;
   tint_symbol[1][0][0] = (*(tint_symbol_1))[0][1][0];
   return;
 }

test/tint/buffer/uniform/std140/array/mat4x2_f16/to_storage.wgsl.expected.msl

@@ -17,7 +17,7 @@ struct tint_array {
 kernel void f(device tint_array<half4x2, 4>* tint_symbol [[buffer(1)]], const constant tint_array<half4x2, 4>* tint_symbol_1 [[buffer(0)]]) {
   *(tint_symbol) = *(tint_symbol_1);
   (*(tint_symbol))[1] = (*(tint_symbol_1))[2];
-  (*(tint_symbol))[1][0] = half2((*(tint_symbol_1))[0][1]).yx;
+  (*(tint_symbol))[1][0] = (*(tint_symbol_1))[0][1].yx;
   (*(tint_symbol))[1][0][0] = (*(tint_symbol_1))[0][1][0];
   return;
 }

test/tint/buffer/uniform/std140/array/mat4x2_f16/to_workgroup.wgsl.expected.msl

@@ -26,7 +26,7 @@ void f_inner(uint local_invocation_index, threadgroup tint_array<half4x2, 4>* co
   threadgroup_barrier(mem_flags::mem_threadgroup);
   *(tint_symbol) = *(tint_symbol_1);
   (*(tint_symbol))[1] = (*(tint_symbol_1))[2];
-  (*(tint_symbol))[1][0] = half2((*(tint_symbol_1))[0][1]).yx;
+  (*(tint_symbol))[1][0] = (*(tint_symbol_1))[0][1].yx;
   (*(tint_symbol))[1][0][0] = (*(tint_symbol_1))[0][1][0];
 }