wgsl parser: replace almost all sem::Type* with typ::Type where possible

All that remains in the wgsl parser that references sem::Type* are
the register_constructed funcs/types, and adding an ast::ExternalTexture
type.

Also:

* Added specialization of OperatorArrow for type::TypePairs that returns
the value by reference so that operator-> can chain properly.

* In a few places where we expect the type to implicitly cast to a
pointer for a bool expression, e.g. `if (type)` or `TINT_ASSERT(type)`,
I added access to the `.sem` member of the TypePair. I tried adding an
implicit cast to bool, but this results in ambiguity in gtest for
equality comparisons.

* Constified more type pointers in type nodes

* Replaced header includes with forward declares in wgsl/parser_impl.h

Bug: tint:724
Change-Id: Ie0875aa4d4a5e830e3466ac40c63cd185f357200
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48881
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/sem/vector_type.cc

@@ -21,7 +21,8 @@ TINT_INSTANTIATE_TYPEINFO(tint::sem::Vector);
 namespace tint {
 namespace sem {
 
-Vector::Vector(Type* subtype, uint32_t size) : subtype_(subtype), size_(size) {
+Vector::Vector(Type const* subtype, uint32_t size)
+    : subtype_(subtype), size_(size) {
   TINT_ASSERT(size_ > 1);
   TINT_ASSERT(size_ < 5);
 }

src/sem/vector_type.h

@@ -28,13 +28,13 @@ class Vector : public Castable<Vector, Type> {
   /// Constructor
   /// @param subtype the vector element type
   /// @param size the number of elements in the vector
-  Vector(Type* subtype, uint32_t size);
+  Vector(Type const* subtype, uint32_t size);
   /// Move constructor
   Vector(Vector&&);
   ~Vector() override;
 
   /// @returns the type of the vector elements
-  Type* type() const { return subtype_; }
+  Type* type() const { return const_cast<Type*>(subtype_); }
   /// @returns the size of the vector
   uint32_t size() const { return size_; }
 
@@ -52,7 +52,7 @@ class Vector : public Castable<Vector, Type> {
   Vector* Clone(CloneContext* ctx) const override;
 
  private:
-  Type* const subtype_;
+  Type const* const subtype_;
   uint32_t const size_;
 };