Start populating named entities in relevant locations. (#919)

This starts detecting naming collisions as a consequence of being able to determine when the name is declared twice in a given scope.



Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/static_scope.cpp

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "executable_semantics/ast/static_scope.h"
+
+#include "executable_semantics/common/error.h"
+
+namespace Carbon {
+
+void StaticScope::Add(std::string name,
+                      Nonnull<const NamedEntityInterface*> entity) {
+  if (!declared_names_.insert({name, entity}).second) {
+    FATAL_COMPILATION_ERROR(entity->source_loc())
+        << "Duplicate name `" << name << "` also found at "
+        << declared_names_[name]->source_loc();
+  }
+}
+
+}  // namespace Carbon