resolver: Fix constant propagation for POC

Pipeline overidable constants are not compile-time constant.
If a module-scope const has an [[override]] decoration, do not assign
the constant value to it, as this will propagate, and the constant value
may become inlined in places that should be overridable.

Also: Rename sem::GlobalVariable::IsPipelineConstant() to
IsOverridable() to make it clearer that this is not a compile-time known
value. Add SetIsOverridable() so we can correctly set the
IsOverridable() flag even when there isn't an ID.

Change-Id: I5ede9dd180d5ff1696b3868ea4313fc28f93af4b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69140
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/sem/variable.cc

@@ -61,8 +61,7 @@ GlobalVariable::GlobalVariable(const ast::Variable* declaration,
                                Constant constant_value,
                                sem::BindingPoint binding_point)
     : Base(declaration, type, storage_class, access, std::move(constant_value)),
-      binding_point_(binding_point),
-      is_pipeline_constant_(false) {}
+      binding_point_(binding_point) {}
 
 GlobalVariable::~GlobalVariable() = default;
 

src/sem/variable.h

@@ -129,22 +129,27 @@ class GlobalVariable : public Castable<GlobalVariable, Variable> {
   /// @returns the resource binding point for the variable
   sem::BindingPoint BindingPoint() const { return binding_point_; }
 
-  /// @returns the pipeline constant ID associated with the variable
-  uint16_t ConstantId() const { return constant_id_; }
-
   /// @param id the constant identifier to assign to this variable
   void SetConstantId(uint16_t id) {
     constant_id_ = id;
-    is_pipeline_constant_ = true;
+    is_overridable_ = true;
   }
 
-  /// @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
+  uint16_t ConstantId() const { return constant_id_; }
+
+  /// @param is_overridable true if this is a pipeline overridable constant
+  void SetIsOverridable(bool is_overridable = true) {
+    is_overridable_ = is_overridable;
+  }
+
+  /// @returns true if this is pipeline overridable constant
+  bool IsOverridable() const { return is_overridable_; }
 
  private:
   const sem::BindingPoint binding_point_;
 
-  bool is_pipeline_constant_ = false;
+  bool is_overridable_ = false;
   uint16_t constant_id_ = 0;
 };