Remove restrictions on expanding GLR stacks in C++.

    * data/glr.c:  Remove tests for __cplusplus related to definition of
    YYSTACKEXPANDABLE.
    * doc/bison.texinfo: Rewrite paragraph saying C++ stacks are not
    expandable to instead indicate conditions that prevent their expansion
    in C++.  Reorganize section on GLR semantic values a bit. Remove
    discussion of YYLLOC_DEFAULT. Mention restrictions to POD data.