Constification of parse_linespec and fallout:
https://sourceware.org/ml/gdb-patches/2013-09/msg01017.html
https://sourceware.org/ml/gdb-patches/2013-09/msg01018.html
https://sourceware.org/ml/gdb-patches/2013-09/msg01019.html
https://sourceware.org/ml/gdb-patches/2013-09/msg01020.html