Test target link information invalidation

We test this by adding export(TARGETS) to the LinkLanguage test to
export the executable before the library is linked to it.  Since
export(TARGETS) computes the link interface of the target (so that it
can export it), this ensures that the information is recomputed after
the link library is added.

Tests/LinkLanguage/CMakeLists.txt

@@ -3,6 +3,11 @@ project(LinkLanguage C CXX)
 
 add_library(foo STATIC foo.cxx)
 add_executable(LinkLanguage LinkLanguage.c)
+
+# Export the target now to compute its link interface and implementation.
+# This tests that the link info is recomputed after the library is linked.
+export(TARGETS LinkLanguage FILE LinkLanguageTargets.cmake)
+
 target_link_libraries(LinkLanguage foo)
 
 # CMake should now automatically choose CXX for linking, so we need