* tool/transform_mjit_header.rb (MJITHeader.check_code): read
output and errors than discarding errors. also cl.exe prints
the source file name which is a garbage at this time.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62788 b2dd03c8-39d4-4d8f-98ff-823fe69b080e