Omit unused implicit argument if return type is immediate.
Add _RUST_STAGE0 #ifdefs
Added _RUST_STAGEN guard to rust_call_tydesc_glue
Implementing suggestions from @nikomatsakis
auto merge of #6731 : thomaslee/rust/issue-6575, r=pcwalton

Fix for #6575. In the trans phase, rustc emits code for a function parameter that goes completely unused in the event the return type of the function in question happens to be an immediate.

This patch modifies rustc & parts of rustrt to ensure that the vestigial parameter is no longer present in compiled code.