Discard MTWT & interner tables from TLD after they stop being useful.
rustc: put ty_trait behind some indirection.

This reduces ty::sty from 160 bytes to just 112, and some measurements
eddyb made suggest that the ty_trait variant occurs very
rarely (e.g. ~1% of all sty instances) hence this will result in a large
memory saving, and the cost of the indirection is unlikely to be an
issue.
rustc: put ty_closure behind some indirection.

This reduces the size of sty from 112 to 96; like with the ty_trait
variant, this variant of sty occurs rarely (~1%) so the benefits are
large and the costs small.
auto merge of #12770 : eddyb/rust/drop-tld, r=cmr

Sadly, this seems to make memory usage worse (unless `Vec<T>` makes it worse and this PR doesn't change that much, which is entirely possible).
auto merge of #13013 : huonw/rust/tytrait, r=cmr

These variants occur rarely but inflate the whole enum for the other variants, leaving a lot of wasted space. In total this reduces `ty::sty` from 160 bytes to 96 (on a 64-bit platform).

After this, `ty_struct` and `ty_enum` are the largest variants, with the 80-byte `substs` being the major contributor.