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]} {