validation: function scope variable store type must be constructible

- function scope variable store type must be constructible
- add IsConstructible() to sem::atomic

Bug: tint:1069
Change-Id: Ib0616b486ecf278dbdd99640dc4ede7f3007feb8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60120
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Sarah Mashayekhi <sarahmashay@google.com>
Auto-Submit: Sarah Mashayekhi <sarahmashay@google.com>

src/sem/atomic_type.cc

@@ -46,6 +46,10 @@ uint32_t Atomic::Align() const {
   return subtype_->Align();
 }
 
+bool Atomic::IsConstructible() const {
+  return false;
+}
+
 Atomic::Atomic(Atomic&&) = default;
 
 Atomic::~Atomic() = default;

src/sem/atomic_type.h

@@ -50,6 +50,10 @@ class Atomic : public Castable<Atomic, Type> {
   /// @returns the alignment in bytes of the type.
   uint32_t Align() const override;
 
+  /// @returns true if constructible as per
+  /// https://gpuweb.github.io/gpuweb/wgsl/#constructible-typesd
+  bool IsConstructible() const override;
+
  private:
   sem::Type const* const subtype_;
 };