PR fortran/95088 - Buffer overflows with PDTs, submodules and long symbols

PR fortran/95088 - Buffer overflows with PDTs, submodules and long symbols

With PDTs (parameterized derived types) and submodules, name mangling
results in variably long internal symbols.  Instead of using a fixed-size
intermediate buffer, which is actually not really needed, just use a
pointer to strings.

2020-06-14  Harald Anlauf  <anlauf@gmx.de>

gcc/fortran/
	PR fortran/95088
	* class.c (get_unique_type_string): Replace use of fixed size
	buffer by internally passing a pointer to strings.

(cherry picked from commit 3ee9d82484bdc0ae5b1b21f4a3d8ff46d40e36fd)