Fix Ada crash with .debug_types

Fix Ada crash with .debug_types

PR ada/25875 concerns a gdb crash when gdb.ada/arr_enum_idx_w_gap.exp
is run using the .debug_types board.

The problem turns out to be caused by weird compiler output.  In this
test, the compiler emits a top-level type that refers to an
enumeration type which is nested in a function.  However, this
function is just a declaration.

This results in gdb calling read_enumeration_type for the enum type,
but process_enumeration_scope is never called, yielding an enum with
no fields.  This causes the crash.

This patch fixes the problem by arranging to create the enum fields in
read_enumeration_type.

Tested on x86-64 Fedora 30.

gdb/ChangeLog
2020-04-29  Tom Tromey  <tromey@adacore.com>

	PR ada/25875:
	* dwarf2/read.c (update_enumeration_type_from_children): Compute
	type fields here.
	(read_enumeration_type): Call
	update_enumeration_type_from_children later.  Update comments.
	(process_enumeration_scope): Don't create type fields.