[Ada] Fix ICE on renaming with discriminated tagged type

[Eric Botcazou]

This is a regression present on the mainline and 4.7 branch.  The compiler 
aborts on the renaming of an object with a derived discriminated tagged type 
converted to the parent type, if the parent type is itself derived from a root 
discriminated tagged type and the renaming is later used as a prefix in the 
selection of a component inherited from the root type.

Tested on i586-suse-linux, applied on mainline and 4.7 branch.


2012-07-03  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/utils2.c (build_simple_component_ref): Do not look
	through an extension if the type contains a placeholder.


2012-07-03  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/discr37.ad[sb]: New test.


-- 
Eric Botcazou
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