Fix LTO bootstrap failure with -Werror=lto-type-mismatch

Fix LTO bootstrap failure with -Werror=lto-type-mismatch

In GNAT's implementation model, using convention C (or C_Pass_By_Copy) has
no effect on the internal representation of types since the representation
is identical to that of C by default.  It's even counter-productive given
the implementation advice listed in B.3(63-71) so the interface between the
front-end and gigi does not use it and instead uses structurally identical
types on both sides.

gcc/ada
	PR ada/117038
	* fe.h (struct c_array): Add 'const' to declaration of pointer.
	(C_Source_Buffer): Use consistent formatting.
	* par-ch3.adb (P_Component_Items): Properly set Aliased_Present on
	access definition.
	* sinput.ads: Remove clause for Interfaces.C.
	(C_Array): Change type of Length to Integer and make both components
	aliased.  Remove Convention aspect.
	(C_Source_Buffer): Remove all aspects.
	* sinput.adb (C_Source_Buffer): Adjust to above change.