[Ada] Crash on return of raise expression

[Ada] Crash on return of raise expression

This patch fixes an issue whereby the compiler regarded assignments to limited
that consisted of raise expressions to be a compile-time error during
expansion.

2018-05-24  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Ignore raising an error in
	expansion for limited tagged types when the node to be expanded is a
	raise expression due to it not representing a valid object.
	* exp_ch5.adb (Expand_N_Assignment_Statement): Add exception to error
	message regarding assignments to limited types to ignore genereated
	code.

gcc/testsuite/

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

From-SVN: r260654