gnu: rust: use llvm@8

* gnu/packages/llvm.scm (llvm): Rename to...
(llvm-8): ...this.
(llvm): New variable.
* gnu/packages/rust.scm (rust)[inputs]: Use llvm-8

gnu/packages/llvm.scm

@@ -48,7 +48,7 @@
   #:use-module (gnu packages python)
   #:use-module (gnu packages xml))
 
-(define-public llvm
+(define-public llvm-8
   (package
     (name "llvm")
     (version "8.0.0")
@@ -99,6 +99,8 @@ languages is in development.  The compiler infrastructure includes mirror sets
 of programming tools as well as libraries with equivalent functionality.")
     (license license:ncsa)))
 
+(define-public llvm llvm-8)
+
 (define* (clang-runtime-from-llvm llvm hash
                                   #:optional (patches '()))
   (package

gnu/packages/rust.scm

@@ -1056,6 +1056,9 @@ jemalloc = \"" jemalloc "/lib/libjemalloc_pic.a" "\"
            "0bbizy6b7002v1rdhrxrf5gijclbyizdhkglhp81ib3bf5x66kas")))
     (package
       (inherit base-rust)
+      (inputs
+       (alist-replace "llvm" (list llvm-8)
+                      (package-inputs base-rust)))
       (arguments
        (substitute-keyword-arguments (package-arguments base-rust)
          ((#:phases phases)