tint->dawn: Shuffle source tree in preperation of merging repos

docs/    -> docs/tint/
fuzzers/ -> src/tint/fuzzers/
samples/ -> src/tint/cmd/
src/     -> src/tint/
test/    -> test/tint/

BUG=tint:1418,tint:1433

Change-Id: Id2aa79f989aef3245b80ef4aa37a27ff16cd700b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80482
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

test/tint/shader_io/shared_struct_helper_function.wgsl

@@ -0,0 +1,18 @@
+struct VertexOutput {
+  @builtin(position) pos : vec4<f32>;
+  @location(0) @interpolate(flat) loc0 : i32;
+};
+
+fn foo(x : f32) -> VertexOutput {
+  return VertexOutput(vec4<f32>(x, x, x, 1.0), 42);
+}
+
+@stage(vertex)
+fn vert_main1() -> VertexOutput {
+  return foo(0.5);
+}
+
+@stage(vertex)
+fn vert_main2() -> VertexOutput {
+  return foo(0.25);
+}