Test OBJECT library language propagation

Teach the RunCMake.ObjectLibrary test to verify that languages used in
an OBJECT library are propagated to targets that use it so that the
languages can be included in link analysis.

Tests/RunCMake/ObjectLibrary/ExportLanguages.cmake

@@ -0,0 +1,15 @@
+enable_language(CXX)
+add_library(A OBJECT a.cxx)
+add_library(B STATIC a.c $<TARGET_OBJECTS:A>)
+
+# Verify that object library languages are propagated.
+export(TARGETS B NAMESPACE Exp FILE BExport.cmake)
+include(${CMAKE_CURRENT_BINARY_DIR}/BExport.cmake)
+get_property(configs TARGET ExpB PROPERTY IMPORTED_CONFIGURATIONS)
+foreach(c ${configs})
+  get_property(langs TARGET ExpB PROPERTY IMPORTED_LINK_INTERFACE_LANGUAGES_${c})
+  list(FIND langs CXX pos)
+  if(${pos} LESS 0)
+    message(FATAL_ERROR "Target export does not list object library languages.")
+  endif()
+endforeach()

Tests/RunCMake/ObjectLibrary/RunCMakeTest.cmake

@@ -6,6 +6,7 @@ run_cmake(BadSourceExpression3)
 run_cmake(BadObjSource1)
 run_cmake(BadObjSource2)
 run_cmake(Export)
+run_cmake(ExportLanguages)
 run_cmake(Import)
 run_cmake(Install)
 run_cmake(LinkObjLHS)

Tests/RunCMake/ObjectLibrary/a.cxx

@@ -0,0 +1 @@
+extern "C" int acxx(void) { return 0; }