[flang][Parser] Convert applyMem to invoke non-void members

Currently applyMem(f, a, ...) calls a.f(...), but still returns the
result of parser a. This is in contrast to applyFunction(f, a, ...),
which returns the result of calling f(a, ...).

The use case for this is being able to parse quoted or unquoted
strings, and store the result as std::string:

```
construct<IdentifierOrString>(
  (space >> charLiteralConstantWithoutKind) ||
  applyMem(&Name::ToString, Parser<Name>{})) // Parser<Name>{}.ToString()
```

The applyMem combinator is currently unused.

Author: kparzysz

flang/docs/ParserCombinators.md

@@ -141,7 +141,7 @@ collect the values that they return.
 * `applyLambda([](&&x){}, p1, p2, ...)` is the same thing, but for lambdas
   and other function objects.
 * `applyMem(mf, p1, p2, ...)` is the same thing, but invokes a member
-  function of the result of the first parser for updates in place.
+  function of the result of the first parser.
 
 ### Token Parsers
 Last, we have these basic parsers on which the actual grammar of the Fortran

flang/lib/Parser/basic-parsers.h

@@ -580,11 +580,11 @@ template <typename PA> inline constexpr auto defaulted(PA p) {
 // applyLambda(f, ...) is the same concept extended to std::function<> functors.
 // It is not constexpr.
 //
-// Member function application is supported by applyMem(f, a).  If the
-// parser a succeeds and returns some value ax, the result is that returned
-// by ax.f().  Additional parser arguments can be specified to supply their
-// results to the member function call, so applyMem(f, a, b) succeeds if
-// both a and b do so and returns the result of calling ax.f(std::move(bx)).
+// Member function application is supported by applyMem(&C::f, a).  If the
+// parser a succeeds and returns some value ax of type C, the result is that
+// returned by ax.f().  Additional parser arguments can be specified to supply
+// their results to the member function call, so applyMem(&C::f, a, b) succeeds
+// if both a and b do so and returns the result of calling ax.f(std::move(bx)).
 
 // Runs a sequence of parsers until one fails or all have succeeded.
 // Collects their results in a std::tuple<std::optional<>...>.
@@ -654,39 +654,31 @@ inline /* not constexpr */ auto applyLambda(
 }
 
 // Member function application
-template <typename OBJPARSER, typename... PARSER> class AMFPHelper {
-  using resultType = typename OBJPARSER::resultType;
-
-public:
-  using type = void (resultType::*)(typename PARSER::resultType &&...);
-};
-template <typename OBJPARSER, typename... PARSER>
-using ApplicableMemberFunctionPointer =
-    typename AMFPHelper<OBJPARSER, PARSER...>::type;
-
-template <typename OBJPARSER, typename... PARSER, std::size_t... J>
-inline auto ApplyHelperMember(
-    ApplicableMemberFunctionPointer<OBJPARSER, PARSER...> mfp,
-    ApplyArgs<OBJPARSER, PARSER...> &&args, std::index_sequence<J...>) ->
-    typename OBJPARSER::resultType {
-  ((*std::get<0>(args)).*mfp)(std::move(*std::get<J + 1>(args))...);
-  return std::get<0>(std::move(args));
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER,
+    std::size_t... J>
+inline auto ApplyHelperMember(MEMFUNC mfp,
+    ApplyArgs<OBJPARSER, PARSER...> &&args, std::index_sequence<J...>) {
+  return ((*std::get<0>(args)).*mfp)(std::move(*std::get<J + 1>(args))...);
 }
 
-template <typename OBJPARSER, typename... PARSER> class ApplyMemberFunction {
-  using funcType = ApplicableMemberFunctionPointer<OBJPARSER, PARSER...>;
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER>
+class ApplyMemberFunction {
+  static_assert(std::is_member_function_pointer_v<MEMFUNC>);
+  using funcType = MEMFUNC;
 
 public:
-  using resultType = typename OBJPARSER::resultType;
+  using resultType =
+      std::invoke_result_t<MEMFUNC, typename OBJPARSER::resultType, PARSER...>;
+
   constexpr ApplyMemberFunction(const ApplyMemberFunction &) = default;
-  constexpr ApplyMemberFunction(funcType f, OBJPARSER o, PARSER... p)
+  constexpr ApplyMemberFunction(MEMFUNC f, OBJPARSER o, PARSER... p)
       : function_{f}, parsers_{o, p...} {}
   std::optional<resultType> Parse(ParseState &state) const {
     ApplyArgs<OBJPARSER, PARSER...> results;
     using Sequence1 = std::index_sequence_for<OBJPARSER, PARSER...>;
     using Sequence2 = std::index_sequence_for<PARSER...>;
     if (ApplyHelperArgs(parsers_, results, state, Sequence1{})) {
-      return ApplyHelperMember<OBJPARSER, PARSER...>(
+      return ApplyHelperMember<MEMFUNC, OBJPARSER, PARSER...>(
           function_, std::move(results), Sequence2{});
     } else {
       return std::nullopt;
@@ -698,11 +690,11 @@ template <typename OBJPARSER, typename... PARSER> class ApplyMemberFunction {
   const std::tuple<OBJPARSER, PARSER...> parsers_;
 };
 
-template <typename OBJPARSER, typename... PARSER>
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER>
 inline constexpr auto applyMem(
-    ApplicableMemberFunctionPointer<OBJPARSER, PARSER...> mfp,
-    const OBJPARSER &objParser, PARSER... parser) {
-  return ApplyMemberFunction<OBJPARSER, PARSER...>{mfp, objParser, parser...};
+    MEMFUNC memfn, const OBJPARSER &objParser, PARSER... parser) {
+  return ApplyMemberFunction<MEMFUNC, OBJPARSER, PARSER...>{
+      memfn, objParser, parser...};
 }
 
 // As is done with function application via applyFunction() above, class