gdb/fortran: Handle gdbarch_floatformat_for_type returning nullptr

gdb/fortran: Handle gdbarch_floatformat_for_type returning nullptr

In this commit:

  commit 34d11c682fd96c7dbe3ebd6cd9033e65d51ec7a3
  Date:   Fri May 3 15:23:55 2019 +0100

      gdb/fortran: Use floatformats_ia64_quad for fortran 16-byte floats

GDB was changed such that the Fortran's 16-byte float format was
obtained by calling gdbarch_floatformat_for_type instead of just using
gdbarch_long_double_format as it was before.

The problem with this default_floatformat_for_type can return NULL in
some cases, and the code introduced in 34d11c682f didn't consider
this.

This commit introduces several alternative strategies for finding a
suitable 16-byte floating point type.  First GDB calls
gdbarch_floatformat_for_type (this was what 34d11c682f added), if this
returns null GDB will use gdbarch_long_double_format if it is the
correct size (this was the format used before 34d11c682f).  Finally,
if neither of the above provides a suitable type then GDB will create
a new dummy type.

This final dummy type is unlikely to provide an correct debug
experience as far as examining the 16-byte floats, but it should
prevent GDB crashing.

gdb/ChangeLog:

	PR gdb/18644:
	* f-lang.c (build_fortran_types): Handle the case where
	gdbarch_floatformat_for_type returns a nullptr.