refactor(irgen): 完善irgen代码和文档，提升扩展兼容性

include/sem/Sema.h

@@ -7,20 +7,20 @@
 
 class SemanticContext {
  public:
-  void BindVarUse(SysYParser::PrimaryContext* use,
-                  SysYParser::VarDeclContext* decl) {
+  void BindVarUse(SysYParser::VarContext* use,
+                  SysYParser::VarDefContext* decl) {
     var_uses_[use] = decl;
   }
 
-  SysYParser::VarDeclContext* ResolveVarUse(
-      const SysYParser::PrimaryContext* use) const {
+  SysYParser::VarDefContext* ResolveVarUse(
+      const SysYParser::VarContext* use) const {
     auto it = var_uses_.find(use);
     return it == var_uses_.end() ? nullptr : it->second;
   }
 
  private:
-  std::unordered_map<const SysYParser::PrimaryContext*,
-                     SysYParser::VarDeclContext*>
+  std::unordered_map<const SysYParser::VarContext*,
+                     SysYParser::VarDefContext*>
       var_uses_;
 };