Add texture_external to inspector

Adds texture_external to the inspector, allowing us to recognize the
type and return provide binding information. Includes a basic test.

Bug: Dawn:728
Change-Id: Ib0f39998359dc22a530ad222141229f9ba30552f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51161
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/sem/function.cc

@@ -16,6 +16,7 @@
 
 #include "src/ast/function.h"
 #include "src/sem/depth_texture_type.h"
+#include "src/sem/external_texture_type.h"
 #include "src/sem/multisampled_texture_type.h"
 #include "src/sem/sampled_texture_type.h"
 #include "src/sem/storage_texture_type.h"
@@ -168,6 +169,24 @@ Function::VariableBindings Function::ReferencedDepthTextureVariables() const {
   return ret;
 }
 
+Function::VariableBindings Function::ReferencedExternalTextureVariables()
+    const {
+  VariableBindings ret;
+
+  for (auto* var : ReferencedModuleVariables()) {
+    auto* unwrapped_type = var->Type()->UnwrapAccess();
+    auto* external_texture = unwrapped_type->As<sem::ExternalTexture>();
+    if (external_texture == nullptr) {
+      continue;
+    }
+
+    if (auto binding_point = var->Declaration()->binding_point()) {
+      ret.push_back({var, binding_point});
+    }
+  }
+  return ret;
+}
+
 bool Function::HasAncestorEntryPoint(Symbol symbol) const {
   for (const auto& point : ancestor_entry_points_) {
     if (point == symbol) {

src/sem/function.h

@@ -135,9 +135,14 @@ class Function : public Castable<Function, CallTarget> {
 
   /// Retrieves any referenced depth texture variables. Note, the variables
   /// must be decorated with both binding and group decorations.
-  /// @returns the referenced storage textures
+  /// @returns the referenced depth textures
   VariableBindings ReferencedDepthTextureVariables() const;
 
+  /// Retrieves any referenced external texture variables. Note, the variables
+  /// must be decorated with both binding and group decorations.
+  /// @returns the referenced external textures
+  VariableBindings ReferencedExternalTextureVariables() const;
+
   /// Checks if the given entry point is an ancestor
   /// @param sym the entry point symbol
   /// @returns true if `sym` is an ancestor entry point of this function