Add a flag to control the balance between GNAT encodings and std. DWARF

Add a flag to control the balance between GNAT encodings and std. DWARF

In order to accomodate the debugger's support evolution for "new" DWARF
constructs, we need to have an flag that controls the amount of GNAT
encodings/standard DWARF information that is emitted in the debug info.
Propagate this new parameter into the Ada front-end.

gcc/ChangeLog:

	* common.opt (gnat_encodings): New variable
	(dwarf_gnat_encodings): New enum type.
	(fgnat_encodings): New option.
	* flag-types.h (enum dwarf_gnat_encodings): New.

gcc/ada/ChangeLog:

	* gcc-interface/misc.c (gnat_encodings): Undefine macro and
	declare a global variable.
	(gnat_post_options): Initialize this global from options.

From-SVN: r231761