g++.exp: Tweak handling of additional source files.

g++.exp: Tweak handling of additional source files.

	* lib/g++.exp: Tweak handling of additional source files.
	* g++.dg/special/conpr-2.C: Use dg-gpp-additional-sources.
	* g++.dg/special/conpr-2a.C: Rename to ...
	* g++.dg/special/conpr-2a.cc: ... this.
	* g++.dg/special/conpr-3.C: Use dg-gpp-additional-sources.
	* g++.dg/special/conpr-3a.C: Rename to ...
	* g++.dg/special/conpr-3a.cc: This.
	* g++.dg/special/conpr-3b.C: Rename to ...
	* g++.dg/special/conpr-3b.cc: This.
	* g++.dg/special/conpr-4.C: New test.
	* g++.dg/special/ecos.exp: Rewrite to use ordinary dg driver.

From-SVN: r66858