Fix linker error:
Undefined symbols for architecture x86_64:
"lldb_private::ClangHighlighter::ClangHighlighter()", referenced from:
    lldb_private::ObjCLanguage::ObjCLanguage() in liblldb-core.a(ObjCLanguage.o)
    lldb_private::CPlusPlusLanguage::CPlusPlusLanguage() in liblldb-core.a(CPlusPlusLanguage.o)
    lldb_private::ObjCPlusPlusLanguage::ObjCPlusPlusLanguage() in liblldb-core.a(ObjCPlusPlusLanguage.o)
"vtable for lldb_private::ClangHighlighter", referenced from:
    lldb_private::ClangHighlighter::~ClangHighlighter() in liblldb-core.a(ObjCLanguage.o)
    lldb_private::ClangHighlighter::~ClangHighlighter() in liblldb-core.a(CPlusPlusLanguage.o)
    lldb_private::ClangHighlighter::~ClangHighlighter() in liblldb-core.a(ObjCPlusPlusLanguage.o)