Merge pull request #591 from hzeller/virtual-override

Consistent use of 'override' for virtual methods in derived classes.

diff --cc backends/json/json.cc
Simple merge

diff --cc frontends/ast/ast.h
Simple merge

diff --cc frontends/blif/blifparse.cc
Simple merge

diff --cc frontends/verific/verific.cc
Simple merge

diff --cc kernel/yosys.cc
Simple merge

diff --cc passes/techmap/extract_reduce.cc
Simple merge