rust: Downgrade to llvm-4.0

[raimue]

The update to 1.24.0 in 4b2c24c erroneously switched to LLVM 5.0, which
is still not supported by this rust version. Compiling with rustc will
produce "invalid expression" messages.

See: rust-lang/rust#43370
See: rust-lang/rust#47464

Type(s)

• bugfix
• enhancement
• security fix

Tested on

I did not test to build it.

Verification

Have you

• followed our Commit Message Guidelines?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?

• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers:
@g5pw for port rust.

[macportsbot]

Travis Build #1808 Errored.

Lint results

--->  Verifying Portfile for rust
--->  0 errors and 0 warnings found.

The build timed out.

[seanfarley]

Looks ok to me.

[pmetzger]

The rust port is sufficiently important, and the fix is sufficiently simple, that I'm wondering if this falls under the "A critical port is broken that affects many users" exception to committing to a port that isn't openmaintainer in less than 72 hours.

@g5pw any comments? Also, would you be willing to make this "openmaintainer" so there is less ambiguity about fixing small issues going forward?

[g5pw]

No comments, thanks for the pull request, sorry for the delay.

[pmetzger]

@g5pw What do you think of making rust openmaintainer?

[g5pw]

It's fine by me, as my time for MacPorts seems to be constantly shrinking :/