Fix for pr3539372.
Fix for pr3539372.

This patch matches what was proposed in pr3539372 to fix ivlpp to correctly
handle an escapped \ at the end of a string.

themperek/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog