[Ada] Wrong code with -gnatVa on lock-free protected objects

[Ada] Wrong code with -gnatVa on lock-free protected objects

This patch fixes the handling of validity checks on protected objects
that use the Lock-Free implementation when validity checks are enabled,
previous to this patch the compiler would report improperly that a
condition in a protected operation was always True (when comoipled with
-gnatwa) and would generate incorrect code fhat operation.

2019-07-01  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* checks.adb (Insert_Valid_Check): Do not apply validity check
	to variable declared within a protected object that uses the
	Lock_Free implementation, to prevent unwarranted constant
	folding, because entities within such an object msut be treated
	as volatile.

gcc/testsuite/

	* gnat.dg/prot7.adb, gnat.dg/prot7.ads: New testcase.

From-SVN: r272873