Add support for override keyword and @id attribute

This replaces the @override keyword that was previously used for
pipeline-overridable constants.

Support for pipeline-overridable constants in Dawn is hidden behind
the "disallow-unsafe-APIs" toggle, so we can make this change directly
instead of deprecating and continuing to support the old syntax.

Fixed: tint:1403
Change-Id: I9b2957a1e33b12772bfe449c0f3a31d929f8aa8b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80480
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>

test/var/override/numbered/zero_init/bool.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : bool = bool();
+@id(1234) override o : bool = bool();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/bool.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : bool = bool();
+@id(1234) override o : bool = bool();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/f32.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : f32 = f32();
+@id(1234) override o : f32 = f32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/f32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : f32 = f32();
+@id(1234) override o : f32 = f32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/i32.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : i32 = i32();
+@id(1234) override o : i32 = i32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/i32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : i32 = i32();
+@id(1234) override o : i32 = i32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/u32.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : u32 = u32();
+@id(1234) override o : u32 = u32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {

test/var/override/numbered/zero_init/u32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-@override(1234) let o : u32 = u32();
+@id(1234) override o : u32 = u32();
 
 @stage(compute) @workgroup_size(1)
 fn main() {