Test INTERPROCEDURAL_OPTIMIZATION build feature

This creates an "IPO" test that builds some other tests as subdirectories
while enabling INTERPROCEDURAL_OPTIMIZATION.  See issue #9615.

Tests/CMakeLists.txt

@@ -108,6 +108,7 @@ IF(BUILD_TESTING)
   ADD_TEST_MACRO(PreOrder PreOrder)
   ADD_TEST_MACRO(COnly COnly)
   ADD_TEST_MACRO(CxxOnly CxxOnly)
+  ADD_TEST_MACRO(IPO COnly/COnly)
   ADD_TEST_MACRO(NewlineArgs NewlineArgs)
   ADD_TEST_MACRO(SetLang SetLang)
   ADD_TEST_MACRO(ExternalOBJ ExternalOBJ)

Tests/IPO/CMakeLists.txt

@@ -0,0 +1,7 @@
+cmake_minimum_required (VERSION 2.8)
+project(IPO NONE)
+
+SET_PROPERTY(DIRECTORY PROPERTY INTERPROCEDURAL_OPTIMIZATION 1)
+
+add_subdirectory(../COnly COnly)
+add_subdirectory(../CxxOnly CxxOnly)