[Ada] Crash on improper pragma Weak_External

[Ada] Crash on improper pragma Weak_External

This patch adds a guard on the use of pragma Weak_External. This pragma
affects link-time addresses of entities, and does not apply to types.
Previous to this patch the compiler would abort on a misuse of the
pragma.

2019-07-01  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma, case Weak_External): Pragma only
	applies to entities with run-time addresses, not to types.

gcc/testsuite/

	* gnat.dg/weak3.adb, gnat.dg/weak3.ads: New testcase.

From-SVN: r272876