[Ada] Strengthen Locked flag

[Ada] Strengthen Locked flag

This patch strengthens the Locked flag, by Asserting that it is False on
operations that might cause reallocation.

No change in behavior (except in the presence of compiler bugs), so no
test.

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* table.adb: Assert that the table is not locked when increasing
	Last, even if it doesn't cause reallocation.  In other words,
	assert that on operations that MIGHT cause reallocation.
	* table.ads: Fix comment accordingly.

From-SVN: r274463