fortran: C++ support for generating C prototypes

fortran: C++ support for generating C prototypes

When generating C prototypes for Fortran procedures with the
-fc-prototypes and -fc-prototypes-external options, print a snippet
defining macros for complex types, and add C++ support by suppressing
mangling.

fortran/ChangeLog:

2019-05-12  Janne Blomqvist  <jb@gcc.gnu.org>

	* dump-parse-tree.c (get_c_type_name): Use macros for complex type
	names.
	* parse.c (gfc_parse_file): Define complex macros, add CPP support
	when printing C prototypes.

From-SVN: r271106