[Ada] Crash on object initialization that is call to expression function

[Pierre-Marie de Rodat]

This patch fixes a compiler abort on an object declaration for a
class-wide type whose expression is a call to an expression function
that returns type extension.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-19  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Call): A call to an expression function
	freezes when expander is active, unless the call appears within
	the body of another expression function,

gcc/testsuite/

	* gnat.dg/expr_func9.adb: New testcase.
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