fix msvc passthrough arguments
fix msvc passthrough arguments

#764 and its followon patch, #768, failed to take into account that
passthrough arguments actually need to be passed through to the compiler
invocation: they don't prevent caching, but they do need to be
considered when computing the cache key...and when actually performing
the compilation.

mozilla/sccache,glandium/sccache,mozilla/sccache,glandium/sccache,luser/sccache2,luser/sccache2,glandium/sccache,mozilla/sccache,luser/sccache2,glandium/sccache,luser/sccache2