[Ada] Compiler abort on call to expr. function for default discriminant

[Pierre-Marie de Rodat]

If a discriminant specification has a default that is a call to an
expression function, that function has to be frozen at the point of a
call to the initialization procedure for an object of the record type,
even though the call does not appear to come from source.

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

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

gcc/ada/

	* sem_res.adb (Resolve_Call): Force the freezing of an
	expression function that is called to provide a default value
	for a defaulted discriminant in an object initialization.

gcc/testsuite/

	* gnat.dg/expr_func5.adb: New testcase.
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