[Ada] Mark Ada subprograms and variables referenced from gigi

[Pierre-Marie de Rodat]

In addition to the C bindings automatically generated during the build
process (einfo.h, sinfo.h, snames.h), gigi also makes use of a manually
maintained C interface to the front-end (atree.h, elists.h, fe.h, namet.h,
nlists.h, repinfo.h, scos.h, stringt.h, types.h, uintp.h, urealp.h).

This change makes sure that the Ada side is fully aware of this manually
maintained C interface by adding WARNING lines to the commentary in the
appropriate places, either globally if there is a specific C header file
or individually for the declarations referenced in the fe.h file.

No functional changes.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-12-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* atree.ads, comperr.ads, debug.ads, einfo.ads, elists.ads,
	err_vars.ads, errout.ads, exp_ch11.ads, exp_code.ads,
	exp_dbug.ads, exp_tss.ads, exp_util.ads, lib.ads, namet.ads,
	nlists.ads, opt.ads, repinfo.ads, restrict.ads, scos.ads,
	sem_aggr.ads, sem_aux.ads, sem_eval.ads, sem_util.ads,
	sinfo.ads, sinput.ads, stringt.ads, targparm.ads, types.ads,
	urealp.ads warnsw.ads: Add WARNING line(s) in commentary.
	* atree.h, elists.h, namet.h, nlists.h, repinfo.h, scos.h,
	stringt.h, types.h, uintp.h, urealp.h: Tidy up.
	* fe.h: Likewise.  Document WARNING mark.
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