C++: fix-it hint for missing parentheses

Consider:

  class t1
  {
  public:
    double length () const { return m_length; }
  private:
    double m_length;
  };

missing-parens-fixit.C: In function 'bool test_1(const t1&)':
missing-parens-fixit.C:14:15: error: invalid use of member function
  'double t1::length() const' (did you forget the '()' ?)
   14 |   return inst.length > 0.0;
      |          ~~~~~^~~~~~

This patch adds a fix-it hint for the case where the member function
takes no parameters, suggesting the addition of the parentheses:

   14 |   return inst.length > 0.0;
      |          ~~~~~^~~~~~
      |                     ()

so that an IDE can potentially apply the fix.

gcc/cp/ChangeLog:
	* typeck2.c: Include "gcc-rich-location.h".
	(cxx_incomplete_type_diagnostic): When complaining about possibly
	missing parens, add a fix-it hint if the member function takes no
	additional params.

gcc/ChangeLog:
	* diagnostic-core.h (emit_diagnostic): New decl.
	* diagnostic.c (emit_diagnostic): New overload, taking a
	rich_location *.

gcc/testsuite/ChangeLog:
	* g++.dg/parse/missing-parens-fixit.C: New test.

From-SVN: r266696