Auto merge of #54941 - pnkfelix:issue-21232-reject-partial-reinit, r=nikomatsakis

reject partial init and reinit of uninitialized data

Reject partial initialization of uninitialized structured types (i.e. structs and tuples) and also reject partial *reinitialization* of such types.

Fix #54986

Fix #54499

cc #21232

diff --cc src/librustc/mir/mod.rs
Simple merge