Implement static name resolution (#958)

This doesn't actually use the results of name resolution, but it does verify that they are present.
Also ensures that name resolution and type checking are applied to deduced function parameters and the implicit call to `Main()`.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/static_scope.cpp

@@ -18,4 +18,37 @@ void StaticScope::Add(std::string name, Nonnull<const NamedEntity*> entity) {
   }
 }
 
+auto StaticScope::Resolve(const std::string& name,
+                          SourceLocation source_loc) const
+    -> Nonnull<const NamedEntity*> {
+  std::optional<Nonnull<const NamedEntity*>> result =
+      TryResolve(name, source_loc);
+  if (!result.has_value()) {
+    FATAL_COMPILATION_ERROR(source_loc)
+        << "'" << name << "' is not declared in this scope";
+  }
+  return *result;
+}
+
+auto StaticScope::TryResolve(const std::string& name,
+                             SourceLocation source_loc) const
+    -> std::optional<Nonnull<const NamedEntity*>> {
+  auto it = declared_names_.find(name);
+  if (it != declared_names_.end()) {
+    return it->second;
+  }
+  std::optional<Nonnull<const NamedEntity*>> result;
+  for (Nonnull<const StaticScope*> parent : parent_scopes_) {
+    auto parent_result = parent->TryResolve(name, source_loc);
+    if (parent_result.has_value() && result.has_value() &&
+        *parent_result != *result) {
+      FATAL_COMPILATION_ERROR(source_loc)
+          << "'" << name << "' is ambiguous between " << (*result)->source_loc()
+          << " and " << (*parent_result)->source_loc();
+    }
+    result = parent_result;
+  }
+  return result;
+}
+
 }  // namespace Carbon