Free intermediate translation results as soon as possible

This fixes the recently introduced peak memory usage regression by
freeing the intermediate results as soon as they're not required
anymore instead of keeping them around for the whole compilation
process.

Refs #8077
auto merge of #8092 : dotdash/rust/peak_mem, r=alexcrichton

This fixes the recently introduced peak memory usage regression by
freeing the intermediate results as soon as they're not required
anymore instead of keeping them around for the whole compilation
process.

Refs #8077