Fix InitLLVM argv (#4405)

`args_.push_back(nullptr);` can resize `args_`, invalidating `argv`. The
order needs to be switched.

Disallow creating instances of abstract classes (#4381)

A good first-pass, at least. (abstract adapters are rejected with this
change, though pending further language design discussion)

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

Use @bazel_tools//tools/cpp:malloc instead of defining a library (#4404)

@bazel_tools//tools/cpp:malloc is equivalent and comes from
https://bazel.build/reference/be/c-cpp#cc_binary.malloc. This also
avoids some confusion in the documentation, since while system_malloc
_can_ be used with `malloc`, it has no effect when it's used with
`--custom_malloc`. But, rather than trying to adjust that, maybe we can
just use @bazel_tools//tools/cpp:malloc

Initial, very rough lowering for calls to specific functions and spec…

…ific function declarations. (#4399)

Generate declarations of specific functions on demand. Definitions are
not emitted yet, and I'm using a temporary, known-broken scheme for name
mangling.

Add tracking of lexed comments, with skeletal formatting. (#4385)

In order to format comments, it's helpful if they're tracked. This
tracks them separately from tokens in order to avoid interfering with
parse; it'd be inconvenient if comment tokens could show up in arbitrary
locations, albeit possible to support.

This additionally extracts out the TokenIterator support into a template
in order to generally have it available for IndexBase types. I'm only
adding it for CommentInfo, not sure if we'll want it elsewhere, but this
structure still felt like a good fit.

Remove out-of-date TODO (#4390)

Improve infrastructure for formatting types in diagnostics. (#4374)

Instead of stringifying types in the caller in some cases, add new types
to represent:

- `InstIdAsType`: an `InstId` diagnostic argument that represents a type
expression that should be included in the diagnostic
- `InstIdAsTypeOfExpr`: an `InstId` diagnostic argument that represents
an expression whose type should be included in the diagnostic

For these cases, we can produce more user-friendly descriptions of a
type than we can with a canonicalized `TypeId`. Add comments to
discourage using `TypeId` diagnostic arguments when one of the above can
be used, and move over existing uses where it's straightforward to do
so.

Move type stringification code to its own files and out of `SemIR::File`
to make `File` smaller and to further discourage the direct use of the
stringification logic.

Also update type printing to include the `` ` `` delimiters surrounding
the type. The intent is that we will eventually want to include other
information when formatting a type, like Clang does when printing a
typedef (`'string' (aka 'std::basic_string<char>')`), and such
formatting requires that the diagnostic machinery produces the `` ` ``s
itself.

There are a couple of cases where we really want to format valid Carbon
type syntax directly into a diagnostic, rather than an `aka` or similar,
because the diagnostic text includes part of the type itself, for
example: ``"consider using `partial {0}`"``. For such cases, a `Raw`
form of the diagnostic argument types is added: `TypeIdAsRawType` and
`InstIdAsRawType`. In principle we could instead use ``"consider using
`partial {0:raw}`"``, but our diagnostic machinery isn't set up for
that.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

Disable the Bazel disk cache on CI. (#4375)

This should free up quite a bit of space and even make our builds faster
by avoiding duplicating every output. The syntax for this flag is
counter intuitive, so I've left a comment explaining it.

`where` check stage, step 3: some type checking (#4364)

With this, we now check:

* The left argument to `where` is a facet type
* The right argument of a rewrite (`=`) requirement converts to the type
of the left argument.
* The left argument of an `impls` requirement is a type and the right
argument is a facet type.

No checking is done for `==` constraints yet.

In addition, make the "is facet type" query into its own function and
fix some comments noticed as part of this change.

This change reveals that accessing the members of a facet, like `.Self`,
isn't doing the right thing, and will have to be fixed in a follow-on
PR. Some tests have been adjusted or disabled as a result.

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

`where` check stage, step 3: some type checking (#4364)

With this, we now check:

* The left argument to `where` is a facet type
* The right argument of a rewrite (`=`) requirement converts to the type
of the left argument.
* The left argument of an `impls` requirement is a type and the right
argument is a facet type.

No checking is done for `==` constraints yet.

In addition, make the "is facet type" query into its own function and
fix some comments noticed as part of this change.

This change reveals that accessing the members of a facet, like `.Self`,
isn't doing the right thing, and will have to be fixed in a follow-on
PR. Some tests have been adjusted or disabled as a result.

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>