Avoid assertion failure due to complex type change

Avoid assertion failure due to complex type change

Tankut Baris Aktemur pointed out that the recent series to change how
complex types are handled introduced a regression.

This assert in init_complex_type was firing:

  gdb_assert (TYPE_CODE (target_type) == TYPE_CODE_INT
	      || TYPE_CODE (target_type) == TYPE_CODE_FLT);

The problem was that f-lang.c could call init_complex_type with a type
whose code was TYPE_CODE_ERROR.

It seemed best to me to fix this in f-lang.c, rather than to change
init_complex_type to accept error types.

Tested on x86-64 Fedora 30.  I'm checking this in.

gdb/ChangeLog
2020-04-02  Tom Tromey  <tromey@adacore.com>

	* f-lang.c (build_fortran_types): Use arch_type to initialize
	builtin_complex_s32 in the TYPE_CODE_ERROR case.