rust: Downgrade to llvm-4.0

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

lang/rust/Portfile

@@ -5,7 +5,7 @@ PortGroup           muniversal 1.0
 
 name                rust
 version             1.24.1
-revision            1
+revision            2
 categories          lang devel
 platforms           darwin
 supported_archs     i386 x86_64
@@ -29,7 +29,9 @@ homepage            https://www.rust-lang.org/
 depends_build       path:bin/cmake:cmake \
                     bin:python2.7:python27
 
-depends_lib         port:llvm-5.0
+# Currently LLVM 4.0 is required, see this issue for LLVM 5.0 support:
+# https://github.com/rust-lang/rust/issues/43370 .
+depends_lib         port:llvm-4.0
 
 master_sites        https://static.rust-lang.org/dist
 
@@ -119,7 +121,7 @@ configure.args      --enable-vendor \
                     --default-linker=${configure.cc} \
                     --disable-codegen-tests \
                     --disable-docs \
-                    --llvm-root=${prefix}/libexec/llvm-5.0 \
+                    --llvm-root=${prefix}/libexec/llvm-4.0 \
                     --release-channel=stable
 
 if {![variant_isset universal]} {