[Ada] Spurious crash on expression function as completion with contracts

[Ada] Spurious crash on expression function as completion with contracts

This patch fixes a compiler abort on an expression function that is a
completion of a subprogram with preconditions. The problem is caused by
the presence of types in the precondition that are not frozen when the
subprogram body constructed for the expression function receives the
code that enforces the precondition. These types must be frozen before
the contract is expanded, so the freeze nodes for these types appear in
the proper scope. This is analogous to what is done with type references
that appear in the original expression of the expression function.

2018-08-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch6.adb: Remove Freeze_Expr_Types.
	* freeze.ads, freeze.adb (Freeze_Expr_Types): Moved from
	sem_ch6.adb, and extended to handle other expressions that may
	contain unfrozen types that must be frozen in their proper
	scopes.
	* contracts.adb (Analyze_Entry_Or_Subprogram_Contract): If the
	contract is for the generated body of an expression function
	that is a completion, traverse the expressions for pre- and
	postconditions to freeze all types before adding the contract
	code within the subprogram body.

gcc/testsuite/

	* gnat.dg/expr_func6.adb, gnat.dg/expr_func6.ads: New testcase.

From-SVN: r263734