[Ada] Crash on expression function and tagged types

[Ada] Crash on expression function and tagged types

This patch fixes a compiler abort on an expression function whose
expression includes tagged types that have not been frozen before the
generated body of the function is analyzed, even though that body is
inserted at the end of the current declarative part.

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

gcc/ada/

	* sem_ch6.adb (Analyze_Subprogram_Body_Helper, Mask_Type):
	Refine the handling of freezing types for expression functions
	that are not completions, when analyzing the generated body for
	the function: the body is inserted at the end of the enclosing
	declarative part, and its analysis may freeze types declared in
	the same scope that have not been frozen yet.

gcc/testsuite/

	* gnat.dg/expr_func7.adb, gnat.dg/expr_func7.ads: New testcase.

From-SVN: r263735