Add missing return after error report.
Genex: Make LINK_LANGUAGE report an error when evaluating link libraries.
Genex: Extend EvaluatingLinkLibraries to also check the top target name.

This will allow testing whether we are evaluating the link libraries
of a particular target.
Genex: Report error if a target file is needed to evaluate link libraries.

Constructs such as

 target_link_libraries(foo $<$<STREQUAL:$<TARGET_FILE:foo>,foo.so>:bar>)

segfault before this patch.
Merge topic 'dag-LINKER_LANGUAGE' into next

ff015ee Genex: Report error if a target file is needed to evaluate link libraries.
b58aff9 Genex: Extend EvaluatingLinkLibraries to also check the top target name.
b1c19ce Genex: Make LINK_LANGUAGE report an error when evaluating link libraries.
0e1cb07 Add missing return after error report.