gdb/fortran: Handle dynamic string types when printing types

gdb/fortran: Handle dynamic string types when printing types

After commit:

  commit 8c2e4e0689ea244d0ed979171a3d09c9176b8175
  Date:   Sun Jul 12 22:58:51 2020 -0400

      gdb: add accessors to struct dynamic_prop

An existing bug was exposed in the Fortran type printing code.  When
GDB is asked to print the type of a function that takes a dynamic
string argument GDB will try to read the upper bound of the string.

The read of the upper bound is written as:

    if (type->bounds ()->high.kind () == PROP_UNDEFINED)
      // Treat the upper bound as unknown.
    else
      // Treat the upper bound as known and constant.

However, this is not good enough.  When printing a function type the
dynamic argument types will not have been resolved.  As a result the
dynamic property is not PROP_UNDEFINED, but nor is it constant.

By rewriting this code to specifically check for the PROP_CONST case,
and treating all other cases as the upper bound being unknown we avoid
incorrectly treating the dynamic property as being constant.

gdb/ChangeLog:

	* f-typeprint.c (f_type_print_base): Allow for dynamic types not
	being resolved.

gdb/testsuite/ChangeLog:

	* gdb.fortran/ptype-on-functions.exp: Add more tests.
	* gdb.fortran/ptype-on-functions.f90: Likewise.