[Ada] Change pragma Compile_Time_Error to force compile-time evaluation

[Ada] Change pragma Compile_Time_Error to force compile-time evaluation

2020-05-25  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Document
	changes to pragmas Compile_Time_Error/Compile_Time_Warning.
	* gnat_rm.texi: Regenerate.
	* libgnat/g-bytswa.adb: Change uses of Compile_Time_Error to
	Compile_Time_Warning, as the actual expression may not always be
	known statically.
	* sem_prag.adb (Analyze_Pragma): Handle differently pragma
	Compile_Time_Error in both compilation and in GNATprove mode.
	(Validate_Compile_Time_Warning_Or_Error): Issue an error or
	warning when the expression is not known at compile time.
	* usage.adb: Add missing documentation for warning switches _c
	and _r.
	* warnsw.ads: Update comment.