Ada: Fix ICE in fld_incomplete_type_of when building GtkAda with LTO

This is a regression from GCC 9 present on mainline and all active branches:
the compilation of GtkAda in LTO mode trips on the assertion present in the
fld_incomplete_type_of function about the TYPE_CANONICAL of types pointed to
by pointer (or reference) types.  The problem comes from an oversight in the
update_pointer_to function on gcc-interface, which correctly propagates the
TYPE_CANONICAL of the new pointer type to the old one when there is a new
pointer type, but fails to synthesize it when there is no new pointer type.

gcc/ada/
	PR ada/123060
	* gcc-interface/utils.cc (update_pointer_to): Synthesize a new
	TYPE_CANONICAL for the old pointer type in the case where there
	is no new pointer type.  Likewise for references.

gcc/testsuite/
	* gnat.dg/lto30.ads, gnat.dg/lto30.adb: New test.