Attribute to force generating C-style parser in C++ output source file

[jmillikin]

A parser that is generated with the Bison C parser API might actually be written in C++, and therefore needs to be emitted in a *.cc source file. There is currently no way to specify this with the bison or bison_cc_library rules, and the workaround conlyopts = ["-x", "c++"], is compiler-specific.