wgsl: Replace [[decoration]] with @decoration

Deprecate the old syntax. Migrate everything to the new syntax.

Bug: tint:1382
Change-Id: Ide12b2e927b17dc93b9714c7049090864cc568d3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77260
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: David Neto <dneto@google.com>

test/let/global/global.wgsl

@@ -18,7 +18,7 @@ let v7 = mat3x3<f32>(vec3<f32>(1.0, 1.0, 1.0), vec3<f32>(1.0, 1.0, 1.0), vec3<f3
 let v8 = MyStruct();
 let v9 = MyArray();
 
-[[stage(fragment)]]
-fn main() -> [[location(0)]] vec4<f32> {
+@stage(fragment)
+fn main() -> @location(0) vec4<f32> {
     return vec4<f32>(0.0,0.0,0.0,0.0);
 }

test/let/global/global.wgsl.expected.wgsl

@@ -22,7 +22,7 @@ let v8 = MyStruct();
 
 let v9 = MyArray();
 
-[[stage(fragment)]]
-fn main() -> [[location(0)]] vec4<f32> {
+@stage(fragment)
+fn main() -> @location(0) vec4<f32> {
   return vec4<f32>(0.0, 0.0, 0.0, 0.0);
 }

test/let/inferred/function.wgsl

@@ -33,7 +33,7 @@ fn let_decls() {
     let v15 = ret_MyArray();
 }
 
-[[stage(fragment)]]
-fn main() -> [[location(0)]] vec4<f32> {
+@stage(fragment)
+fn main() -> @location(0) vec4<f32> {
     return vec4<f32>(0.0,0.0,0.0,0.0);
 }

test/let/inferred/function.wgsl.expected.wgsl

@@ -42,7 +42,7 @@ fn let_decls() {
   let v15 = ret_MyArray();
 }
 
-[[stage(fragment)]]
-fn main() -> [[location(0)]] vec4<f32> {
+@stage(fragment)
+fn main() -> @location(0) vec4<f32> {
   return vec4<f32>(0.0, 0.0, 0.0, 0.0);
 }