The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.

Revisions asked by eddyb :
- Renamed of all the occurences of {visit/super}_mir
- Renamed test structures `CachedMir` to `Cached`

Fixing the missing import on `AggregateKind`

Once upon a time (commit 9bd35c07) there were two kinds of
projection: one for places, and one for constants. It therefore made
sense to share the `Projection` struct for both. Although the different
use-cases used different concrete types, sharing was made possible by
type-parameterisation of `Projection`.

Since then, however, the usage of projections in constants has
disappeared, meaning that (forgetting lifetimes for a moment) the
parameterised type is only every instantiated under one guise. So it may
as well be a concrete type.

The `InlineAsm` variant is extremely rare, and `mir::Statement` often
contributes significantly to peak memory usage.

Since lexical MIR borrow check is gone, and validation no longer uses
these, they can be removed.

This commit makes two changes:

First, it updates the dataflow builder to add an init for the place
containing a union if there is an assignment into the field of
that union.

Second, it stops a "use of uninitialized" error occuring when there is an
assignment into the field of an uninitialized union that was previously
initialized. Making this assignment would re-initialize the union, as
tested in `src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr`.
The check for previous initialization ensures that we do not start
supporting partial initialization yet (cc #21232, #54499, #54986).

This reduces allocation counts significantly in a few benchmarks,
reducing instruction counts by up to 2%.

Also "rename" -Zmir-emit-validate to -Zmir-emit-retag, which is just a boolean (yes or no).

E0669 refers to a constraint that cannot be coerced into a single LLVM
value, unfortunately right now this uses the Span for the entire inline
assembly statement, which is less than ideal.

This commit preserves the Span from HIR, which lets us emit the error
using the Span for the operand itself in MIR.
Signed-off-by: NLevente Kurusa <lkurusa@acm.org>

This shrinks StatementKind from 64 bytes to 48 bytes on 64-bit.