1. Move class V3ParseGrammar into V3ParseGrammar.h so editors understand
it as c++ code
2. Fix #line directives in the bison output file
This together enables us to gdb through V3ParseGrammar, verilog.y, and
the bison generated C code step by step, with all source annotations in
the debug info pointing to the right place (e.g.: you will step to the
right place in verilog.y, then step back to the bison generated switch
statement/loop, and then step into calls in V3ParseGrammar as kind of
expected.