connection_notifier: prevent link errors due to variables defined in …

…header

connection_notifier.hh defines a number of template-specialized
variables in a header. This is illegal since you're allowed to
define something multiple times if it's a template, but not if it's
fully specialized. gcc doesn't care but clang notices and complains.

Fix by defining the variiables as inline variables, which are
allowed to have definitions in multiple translation units.

Closes scylladb#7519

connection_notifier.hh

@@ -50,12 +50,12 @@ enum class changed_column {
 };
 
 template <changed_column column> constexpr const char* column_literal = "";
-template <> constexpr const char* column_literal<changed_column::username> = "username";
-template <> constexpr const char* column_literal<changed_column::connection_stage> = "connection_stage";
-template <> constexpr const char* column_literal<changed_column::driver_name> = "driver_name";
-template <> constexpr const char* column_literal<changed_column::driver_version> = "driver_version";
-template <> constexpr const char* column_literal<changed_column::hostname> = "hostname";
-template <> constexpr const char* column_literal<changed_column::protocol_version> = "protocol_version";
+template <> inline constexpr const char* column_literal<changed_column::username> = "username";
+template <> inline constexpr const char* column_literal<changed_column::connection_stage> = "connection_stage";
+template <> inline constexpr const char* column_literal<changed_column::driver_name> = "driver_name";
+template <> inline constexpr const char* column_literal<changed_column::driver_version> = "driver_version";
+template <> inline constexpr const char* column_literal<changed_column::hostname> = "hostname";
+template <> inline constexpr const char* column_literal<changed_column::protocol_version> = "protocol_version";
 
 enum class client_connection_stage {
     established = 0,
@@ -64,9 +64,9 @@ enum class client_connection_stage {
 };
 
 template <client_connection_stage ccs> constexpr const char* connection_stage_literal = "";
-template <> constexpr const char* connection_stage_literal<client_connection_stage::established> = "ESTABLISHED";
-template <> constexpr const char* connection_stage_literal<client_connection_stage::authenticating> = "AUTHENTICATING";
-template <> constexpr const char* connection_stage_literal<client_connection_stage::ready> = "READY";
+template <> inline constexpr const char* connection_stage_literal<client_connection_stage::established> = "ESTABLISHED";
+template <> inline constexpr const char* connection_stage_literal<client_connection_stage::authenticating> = "AUTHENTICATING";
+template <> inline constexpr const char* connection_stage_literal<client_connection_stage::ready> = "READY";
 
 // Representation of a row in `system.clients'. std::optionals are for nullable cells.
 struct client_data {