tint: Optimize sem node lookup

Add a 'NodeID' to each ast::Node which is the sequentially allocated
index of the node. Use this in sem::Info to map the AST node to the
semantic node, instead of using a std::unordered_map.

Optimised very hot code by entirely eliminating map lookups, and
dramatically reducing cache misses (lookups are usually sequentually
ordered).

Timings running
'webgpu:shader,execution,expression,call,builtin,atan2:f32:inputSource="const";vectorize="_undef_"'
with dawn/node, using SwiftShader:

    Without change: 3.22647107s
    With change:    3.10578879s

Bug: tint:1613
Change-Id: I22ec48d933b2e5f9da04494bff4e979e6f7b1982
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96140
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/ast/struct_member_align_attribute.cc b/src/tint/ast/struct_member_align_attribute.cc
index f586e7e..d8ed4fc 100644
--- a/src/tint/ast/struct_member_align_attribute.cc
+++ b/src/tint/ast/struct_member_align_attribute.cc
@@ -23,8 +23,11 @@
 
 namespace tint::ast {
 
-StructMemberAlignAttribute::StructMemberAlignAttribute(ProgramID pid, const Source& src, uint32_t a)
-    : Base(pid, src), align(a) {}
+StructMemberAlignAttribute::StructMemberAlignAttribute(ProgramID pid,
+                                                       NodeID nid,
+                                                       const Source& src,
+                                                       uint32_t a)
+    : Base(pid, nid, src), align(a) {}
 
 StructMemberAlignAttribute::~StructMemberAlignAttribute() = default;