[Ada] Fix ICE related to type freezing

[Ada] Fix ICE related to type freezing

2018-09-26  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* contracts.adb (Expand_Subprogram_Contract,
	Process_Preconditions_For): Apply Freeze_Expr_Types to the
	expression for a precondition of an expression function that is
	a completion, when the completion appears in the private part
	and the declaration it completes is in the visible part of the
	same package.
	* freeze.adb (Freeze_Expr_Types): Do not establish the scope of
	the operation if it is already installed, as will be the case
	when called to analyze the contract oc the subprogram (which
	happens when generating code inside the subprogram body).

From-SVN: r264602