Parse parameters in name qualifiers. (#3988)

Parse the name of a declaration as a sequence of `NameQualifier`s --
which have a name, possibly parameters, and a trailing period --
followed by a name and possibly parameters. This prepares us for parsing
declarations of members of generic classes and similar cases, but
actually supporting such member redeclarations is left to a future
change.

We previously required functions to have parameters, but no longer do,
following the direction of #3848. Cases like namespaces that can't
actually have parameters are now diagnosed in check instead of in parse.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/parse/handle_function.cpp

@@ -9,7 +9,7 @@ namespace Carbon::Parse {
 auto HandleFunctionIntroducer(Context& context) -> void {
   auto state = context.PopState();
   context.PushState(state, State::FunctionAfterParams);
-  context.PushState(State::DeclNameAndParamsAsRequired, state.token);
+  context.PushState(State::DeclNameAndParams, state.token);
 }
 
 auto HandleFunctionAfterParams(Context& context) -> void {