No longer potentially call `mem::uninitialized::<!>()`

Fixes #39432

Previously, the note/message for the source of a lint being the command
line unconditionally named the individual lint, even if the actual
command specified a lint group (e.g., `-D warnings`); here, we take note
of the actual command options so we can be more specific.

This remains in the matter of #36846.

This functionality was missing, and should have existed previously.

Fixes #38780

Fixes #12327.

These two attributes are used to change the entry point into a Rust program, but
for now they're being put behind feature gates until we have a chance to think
about them a little more. The #[start] attribute specifically may have its
signature changed.

This is a breaking change to due the usage of these attributes generating errors
by default now. If your crate is using these attributes, add this to your crate
root:

    #![feature(start)] // if you're using the #[start] attribute
    #![feature(main)]  // if you're using the #[main] attribute

cc #20064

This will temporarily prevent warnings generated from expanding to code that the
test harness itself uses. This solution will require tweaking around the beta
cycle, but it will prevent spurious warnings for now.

Closes #20823

Closes #17732
Closes #18819
Closes #19479
Closes #19631
Closes #19632
Closes #19850
Closes #19883
Closes #20005
Closes #20009
Closes #20389

The privacy pass of the compiler was previously not taking into account the
privacy of foreign items, or bindings to external functions. This commit fixes
this oversight by encoding the visibility of foreign items into the metadata for
each crate.

Any code relying on this will start to fail to  compile and the bindings must be
marked with `pub` to indicate that they can be used externally.

Closes #16725
[breaking-change]

Each test works by rendering the flowgraph for the last identified
block we see in expanded pretty-printed output, and comparing it (via
`diff`) against a checked in "foo.dot-expected.dot" file.

Each test post-processes the output to remove NodeIds ` (id=NUM)` so
that the expected output is somewhat stable (or at least independent
of how we assign NodeIds) and easier for a human to interpret when
looking at the expected output file itself.

----

Test writing style notes:

I usually tried to write the tests in a way that would avoid duplicate
labels in the output rendered flow graph, when possible.

The tests that have string literals "unreachable" in the program text
are deliberately written that way to remind the reader that the
unreachable nodes in the resulting graph are not an error in the
control flow computation, but rather a natural consequence of its
construction.

Signed-off-by: NDaniel Fagnan <dnfagnan@gmail.com>

This infrastructure is meant to support runnings tests that involve various
interesting interdependencies about the types of crates being linked or possibly
interacting with C libraries. The goal of these make tests is to not restrict
them to a particular test runner, but allow each test to run its own tests.

To this end, there is a new src/test/run-make directory which has sub-folders of
tests. Each test requires a `Makefile`, and running the tests constitues simply
running `make` inside the directory. The new target is `check-stageN-rmake`.

These tests will have the destination directory (as TMPDIR) and the local rust
compiler (as RUSTC) passed along to them. There is also some helpful
cross-platform utilities included in src/test/run-make/tools.mk to aid with
compiling C programs and running them.

The impetus for adding this new test suite is to allow various interesting forms
of testing rust linkage. All of the tests initially added are various flavors of
compiling Rust and C with one another as well as just making sure that rust
linkage works in general.

Closes #10434