[Ada] Visibility error on used enumerated type

[Ada] Visibility error on used enumerated type

This patch fixes an issue whereby the freezing of a nested package
containing an enumerated type declaration would cause visibility errors
on literals of such type when a use_all_type_clause for it appears
within the same declarative region.

2018-11-14  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch7.adb (Uninstall_Declarations): Add conditional to avoid
	uninstalling potential visibility during freezing on enumeration
	literals.

gcc/testsuite/

	* gnat.dg/enum5.adb: New testcase.

From-SVN: r266119