MIR based borrow check (opt-in).

One can either use `-Z borrowck-mir` or add the `#[rustc_mir_borrowck]` attribute
to opt into MIR based borrow checking.

Note that regardless of whether one opts in or not, AST-based borrow
check will still run as well.  The errors emitted from AST-based
borrow check will include a "(Ast)" suffix in their error message,
while the errors emitted from MIR-based borrow check will include a
"(Mir)" suffix.

post-rebase: removed check for intra-statement mutual conflict;
replaced with assertion checking that at most one borrow is generated
per statement.

post-rebase: removed dead code: `IdxSet::pairs` and supporting stuff.

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