Move tcx.ccache into just const_eval

According to http://huonw.github.io/isrustfastyet/mem/#012f909, the "const
marking" pass generates about 400MB of extra memory during compilation. This
pass appears to only fill in this ccache map, and the ccache map isn't used
anywhere outside of the const_eval pass. For these reasons, this commit moves
the ccache map into only the const_eval pass so it can be dropped as soon as
const_eval is done, hopefully shaving another 400MB off the peak of compilation.