c++: Add warning about missing 'requires'

c++: Add warning about missing 'requires'

I noticed that concepts-lambda14.C had two useless requires-expressions:

  static_assert(requires { C<T>; });

always succeeds, because C<T> is always a valid expression for any type,
regardless of whether C is satisfied for a particular type.  Presumably the
user means

  static_assert(requires { requires C<T>; });

to make the C<T> a nested-requirement.  Of course,

  static_assert(C<T>);

is much simpler and means the same thing; this is more relevant in the
middle of a longer requires-expression, such as the bug this warning found
in cmcstl2:

  template<class I>
  META_CONCEPT input_iterator =
	  input_or_output_iterator<I> &&
	  readable<I> &&
	  requires(I& i, const I& ci) {
		  typename iterator_category_t<I>;
		  derived_from<iterator_category_t<I>, input_iterator_tag>;
		  i++;
	  };

where 'requires' is missing before 'derived_from'.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wmissing-requires.

gcc/c-family/ChangeLog:

	* c.opt: Add -Wmissing-requires.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_simple_requirement): Warn about missing
	requires.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda14.C: Add expected warnings.