[Ada] Front-end support for OpenACC pragmas

[Ada] Front-end support for OpenACC pragmas

This patch introduces the upper part of the Ada front-end support for a first
set of OpenACC directives (Acc_Kernels, Acc_Loop, Acc_Parallel, Acc_Data) and
their clauses.

The pragmas syntax is documented, pragmas are recognized and checked for
syntactic correctness, keyed on -fopenacc. A couple of new flags are defined
on loop statements in the GNAT tree.

2018-09-26  Olivier Hainque  <hainque@adacore.com>

gcc/ada/

	* opt.ads (OpenAcc_Enabled): New flag. False by default. True
	when OpenACC pragmas are requested to be honored, when -fopenacc
	is found on the command line.
	* back_end.adb (Scan_Compiler_Arguments): Set OpenACC_Enabled if
	-fopenacc is seen on the command line.
	* sinfo.adb, sinfo.ads (Is_OpenAcc_Environment): New
	flag/predicate on Loop statements which embed an Acc_Kernels,
	Acc_Parallel or Acc_Data pragma.
	(Is_OpenAcc_Loop): New flag/predicate on Loop statements which
	embed an Acc_Loop pragma.
	(Set_Is_OpenAcc_Environment, Set_Is_OpenAcc_Loop): Setters for
	the new flags.
	* par-prag.adb (Prag): Handle Acc_Data, Acc_Loop, Acc_Parallel
	and Acc_Kernels pragmas.  Nothing to do here, all handled by
	sem_prag.
	* sem_prag.adb (Acc_First, Acc_Next,
	Validate_Acc_Condition_Clause, Validate_Acc_Data_Clause,
	Validate_Acc_Int_Expr_Clause, Validate_Acc_Int_Expr_List_Clause,
	Validate_Acc_Loop_Collapse, Validate_Acc_Loop_Gang,
	Validate_Acc_Loop_Vector, Validate_Acc_Loop_Worker,
	Validate_Acc_Name_Reduction, Validate_Acc_Size_Expressions): New
	helper for Analyze_Pragma, to handle OpenACC pragmas.
	(Analyze_Pragma): Handle Acc_Data, Acc_Loop, Acc_Parallel and
	Acc_Kernels pragmas.
	* sem_ch5.adb (Disable_Constant): Unset Is_True_Constant on
	variable entity, action for ...
	(Disable_Constants): Helper for Analyze_Loop_Statement, to ...
	(Analyze_Loop_Statement): Disable True_Constant on variables
	referenced within an OpenACC environment.
	* snames.ads-tmpl: Declare Name_Ids for the OpenACC directives
	and clauses we can handle. Remove an exraneous whitespace before
	columns, preventing line length overflow in the generated spec
	with Ids now reaching beyond 999.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Document
	pragma Acc_Parallel, Acc_Loop, Acc_Kernels and Acc_Data.
	* gnat_rm.texi: Regenerate.

From-SVN: r264617