[Ada] Extend the applicability of Thread_Local_Storage to composite types

[Pierre-Marie de Rodat]

This patch allows the GNAT-specific Thread_Local_Storage to be applied
to variables of a composite type initiallized with an aggregate with
static components that requires no elaboration code.

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

2018-05-30  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* freeze.adb (Freeze_Object_Declaration): A pragma Thread_Local_Storage
	is now legal on a variable of composite type initialized with an
	aggregate that is fully static and requires no elaboration code.
	* exp_aggr.adb (Convert_To_Positional): Recognize additional cases of
	nested aggregates that are compile-time static, so they can be used to
	initialize variables declared with Threqd_Local_Storage.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Add documentation on
	Thread_Local_Storage.
	* gnat_rm.texi: Regenerate.

gcc/testsuite/

	* gnat.dg/tls1.adb, gnat.dg/tls1_pkg.ads: New testcase.
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