resolver: Allocate IDs for named pipeline constants

Keep track of any constant IDs specified in the shader, and then
allocate IDs for the remaining constants when creating the semantic
info.

Bug: tint:755
Change-Id: I6a76b1193cac459b62582cde7469b092dde51d5d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50841
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/sem/variable.cc

@@ -26,7 +26,19 @@ namespace sem {
 Variable::Variable(const ast::Variable* declaration,
                    const sem::Type* type,
                    ast::StorageClass storage_class)
-    : declaration_(declaration), type_(type), storage_class_(storage_class) {}
+    : declaration_(declaration),
+      type_(type),
+      storage_class_(storage_class),
+      is_pipeline_constant_(false) {}
+
+Variable::Variable(const ast::Variable* declaration,
+                   const sem::Type* type,
+                   uint16_t constant_id)
+    : declaration_(declaration),
+      type_(type),
+      storage_class_(ast::StorageClass::kNone),
+      is_pipeline_constant_(true),
+      constant_id_(constant_id) {}
 
 Variable::~Variable() = default;
 

src/sem/variable.h

@@ -37,7 +37,7 @@ class VariableUser;
 /// Variable holds the semantic information for variables.
 class Variable : public Castable<Variable, Node> {
  public:
-  /// Constructor
+  /// Constructor for variables and non-overridable constants
   /// @param declaration the AST declaration node
   /// @param type the variable type
   /// @param storage_class the variable storage class
@@ -45,6 +45,14 @@ class Variable : public Castable<Variable, Node> {
            const sem::Type* type,
            ast::StorageClass storage_class);
 
+  /// Constructor for overridable pipeline constants
+  /// @param declaration the AST declaration node
+  /// @param type the variable type
+  /// @param constant_id the pipeline constant ID
+  Variable(const ast::Variable* declaration,
+           const sem::Type* type,
+           uint16_t constant_id);
+
   /// Destructor
   ~Variable() override;
 
@@ -63,11 +71,19 @@ class Variable : public Castable<Variable, Node> {
   /// @param user the user to add
   void AddUser(const VariableUser* user) { users_.emplace_back(user); }
 
+  /// @returns true if this variable is an overridable pipeline constant
+  bool IsPipelineConstant() const { return is_pipeline_constant_; }
+
+  /// @returns the pipeline constant ID associated with the variable
+  uint32_t ConstantId() const { return constant_id_; }
+
  private:
   const ast::Variable* const declaration_;
   const sem::Type* const type_;
   ast::StorageClass const storage_class_;
   std::vector<const VariableUser*> users_;
+  const bool is_pipeline_constant_;
+  const uint16_t constant_id_ = 0;
 };
 
 /// VariableUser holds the semantic information for an identifier expression