ada: Do not issue SPARK legality error if SPARK_Mode ignored

ada: Do not issue SPARK legality error if SPARK_Mode ignored

When pragma Ignore_Pragma(SPARK_Mode) is used, do not issue error
messages related to SPARK legality checking. This facilitates the
instrumentation of code by GNATcoverage.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Fix doc for
	pragma Ignore_Pragma, in the case where it follows another
	configuration pragma that it names, which causes the preceding
	pragma to be ignored after parsing.
	* errout.adb (Should_Ignore_Pragma_SPARK_Mode): New query.
	(SPARK_Msg_N): Do nothing if SPARK_Mode is ignored.
	(SPARK_Msg_NE): Same.
	* gnat-style.texi: Regenerate.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.