[modularize] Fix compilation warning after r372681

[modularize] Fix compilation warning after r372681

In r372681 lang_cxx_11 and lang_cxx_14 were added to LanguageIDs
but they were not handled in the switch in VisitLinkageSpecDecl in
Modularize.cpp so at clang 8 complained with

/data/repo/master/clang-tools-extra/modularize/Modularize.cpp:583:13: error: enumeration values 'lang_cxx_11' and 'lang_cxx_14' not handled in switch [-Werror,-Wswitch]
    switch (D->getLanguage()) {
            ^
1 error generated.

With this patch we now treat lang_cxx, lang_cxx_11 and lang_cxx_14 the
same way in the switch in VisitLinkageSpecDecl.

llvm-svn: 372714