Re-allow dynamic indexing of 'let' arrays and matrices

Spec change: https://github.com/gpuweb/gpuweb/pull/2427
Reverses: tint:867

This reverts and fixes commits:
 b6fdcc54df
 10442eff7d

Added a bunch of end-to-end tests.

Fixed: tint:1352
Change-Id: I34968243bbec1cab838c8ba50a6f027146bbfd06
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/75401
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/sem/constant.h

@@ -126,6 +126,13 @@ class Constant {
     return func(~0);
   }
 
+  /// @param index the index of the scalar value
+  /// @return the value of the scalar `static_cast` to type T.
+  template <typename T>
+  T ElementAs(size_t index) const {
+    return WithScalarAt(index, [](auto val) { return static_cast<T>(val); });
+  }
+
  private:
   const sem::Type* type_ = nullptr;
   const sem::Type* elem_type_ = nullptr;