Fixes bug 34233.

(This has bug has been backported to 0.3.5, but only released in
0.4.3, so it only needs a changes file there.)

This macro defers to __attribute__((fallthrough)) on GCC (and
clang).  Previously we had been using GCC's magic /* fallthrough */
comments, but clang very sensibly doesn't accept those.

Since not all compiler recognize it, we only define it when our
configure script detects that it works.

Part of a fix for 34078.

We now accumulate warning flags in a separate variable,
"TOR_WARNING_FLAGS", and write it to a "warning_flags" file.  Then
we test whether the compiler will accept "@warning_flags": if so, we
put "@warning_flags" in the CFLAGS; if not, we copy the contents of
"$TOR_WARNING_FLAGS" into the CFLAGS.

Closes ticket 28924.

Failing to do on clients was causing TLS 1.3 negotiation to fail.

Fixes bug 28245; bugfix on 0.2.9.15, when we added TLS 1.3 support.

This'll help retain test compatibility until 1.31.0 is released!

This is no longer necessary with upstream rust-lang/rust changes as well
as some local tweaks. Namely:

* The `-fsanitize=address`-style options are now passed via `-C
  link-args` through `RUSTFLAGS`. This obviates the need for the shell
  script.
* The `-C default-linker-libraries`, disabling `-nodefaultlibs`, is
  passed through `RUSTFLAGS`, which is necessary to ensure that
  `-fsanitize=address` links correctly.
* The `-C linker` option is passed to ensure we're using the same C
  compiler as normal C code, although it has a bit of hackery to only
  get the `gcc` out of `gcc -std=c99`

Closes ticket 23846