rustc_mir: don't use upvar_decls in the miri validity checks.
rustc_mir: don't use upvar_decls in the miri validity checks.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust