test: Add E2E test coverage for shader IO

This provides much more complete coverage than what we have in the
unit tests. We now test:
- All builtins, for all stages, both struct and non-struct
- Multiple location attributes for vertex and fragment stages, both
  struct and non-struct
- Mixing builtins and location attributes, whilst mixing struct and
  non-struct
- A few "interesting" cases of IO structs being shared between
  different functions, stages, and with an SSBO variable

There are 7 skipped tests for MSL due to two different MSL bugs which
will be fixed in upcoming patches.

Change-Id: I8b802591762c8ff018e01bf37838551e353162b1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53120
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>

test/shader_io/shared_struct_storage_buffer.wgsl

@@ -0,0 +1,17 @@
+[[block]]
+struct S {
+  [[align(64)]] [[location(0)]] f : f32;
+  [[size(32)]] [[location(1)]] u : u32;
+  [[align(128)]] [[builtin(position)]] v : vec4<f32>;
+};
+
+[[group(0), binding(0)]]
+var<storage> output : [[access(write)]] S;
+
+[[stage(fragment)]]
+fn frag_main(input : S) {
+  let f : f32 = input.f;
+  let u : u32 = input.u;
+  let v : vec4<f32> = input.v;
+  output = input;
+}