ignore whole-range overlapping initializer

ignore whole-range overlapping initializer

When an array is initialized, it may be convenient to first
initialize all entries with some default value via the
'[a ... b]' notation and then override some of these entries
with a non-default value. Unfortunately, this, of course,
is not compatible with the default warning flag '-Woverride-init'.

Fix this by introducing an exception to the usual detection
of overlapping initializers which, only for what concerns this
warning, ignore an '[a ... b]' entry if:
- it is the first one
- it covers the whole range of the array.

If needed, the previous ehaviour can be restored by using a new
warning flag, disabled by default: '-Woverride-init-whole-range'.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>