Convert `@align` to hold an expression.

With the updated WGSL grammar the `@align` attribute takes an
expression instead of a value. This CL updates the internal data
structures to store an expression instead of an integer value.

The parser still only parses integers, they're just turned into
`IntLiteralExpressions` at the moment.

Bug: tint:1633
Change-Id: If34901798ed6ceaced354bc06ae9b6df875b700e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/99980
Reviewed-by: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/program_builder.h

diff --git a/src/tint/program_builder.h b/src/tint/program_builder.h
index 42e428a..5da80d6 100644
--- a/src/tint/program_builder.h
+++ b/src/tint/program_builder.h
@@ -2274,17 +2274,19 @@
 
     /// Creates a ast::StructMemberAlignAttribute
     /// @param source the source information
-    /// @param val the align value
+    /// @param val the align value expression
     /// @returns the align attribute pointer
-    const ast::StructMemberAlignAttribute* MemberAlign(const Source& source, uint32_t val) {
-        return create<ast::StructMemberAlignAttribute>(source, val);
+    template <typename EXPR>
+    const ast::StructMemberAlignAttribute* MemberAlign(const Source& source, EXPR&& val) {
+        return create<ast::StructMemberAlignAttribute>(source, Expr(std::forward<EXPR>(val)));
     }
 
     /// Creates a ast::StructMemberAlignAttribute
-    /// @param val the align value
+    /// @param val the align value expression
     /// @returns the align attribute pointer
-    const ast::StructMemberAlignAttribute* MemberAlign(uint32_t val) {
-        return create<ast::StructMemberAlignAttribute>(source_, val);
+    template <typename EXPR>
+    const ast::StructMemberAlignAttribute* MemberAlign(EXPR&& val) {
+        return create<ast::StructMemberAlignAttribute>(source_, Expr(std::forward<EXPR>(val)));
     }
 
     /// Creates the ast::GroupAttribute