Do not break before "0" in pure virtual function declarations.

Before:
virtual void write(ELFWriter *writer, OwningPtr<FileOutputBuffer> &buffer) =
    0

After:
virtual void write(ELFWriter *writerrr,
                   OwningPtr<FileOutputBuffer> &buffer) = 0;

This addresses llvm.org/PR14815.

To implement this I introduced a line type during parsing and moved the
definition of TokenType out of the struct for increased readability.
Should have done the latter in a separate patch, but it would be hard to
pull apart now.