Make rust_toolchain.llvm_tools consistent with rust_std (#1180)

* Make `rust_toolchain.llvm_tools` consistent with `rust_std`

* Regenerate documentation

Author: UebelAndre

docs/flatten.md

@@ -1120,7 +1120,7 @@ See @rules_rust//rust:repositories.bzl for examples of defining the @rust_cpuX r
 | <a id="rust_toolchain-default_edition"></a>default_edition |  The edition to use for rust_* rules that don't specify an edition.   | String | optional | "2018" |
 | <a id="rust_toolchain-dylib_ext"></a>dylib_ext |  The extension for dynamic libraries created from rustc.   | String | required |  |
 | <a id="rust_toolchain-exec_triple"></a>exec_triple |  The platform triple for the toolchains execution environment. For more details see: https://docs.bazel.build/versions/master/skylark/rules.html#configurations   | String | required |  |
-| <a id="rust_toolchain-llvm_tools"></a>llvm_tools |  LLVM tools that are shipped with the Rust toolchain.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
+| <a id="rust_toolchain-llvm_tools"></a>llvm_tools |  LLVM tools that are shipped with the Rust toolchain.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "0", "opt": "3"} |
 | <a id="rust_toolchain-os"></a>os |  The operating system for the current toolchain   | String | required |  |
 | <a id="rust_toolchain-rust_doc"></a>rust_doc |  The location of the <code>rustdoc</code> binary. Can be a direct source or a filegroup containing one item.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |

docs/rust_repositories.md

@@ -95,7 +95,7 @@ See @rules_rust//rust:repositories.bzl for examples of defining the @rust_cpuX r
 | <a id="rust_toolchain-default_edition"></a>default_edition |  The edition to use for rust_* rules that don't specify an edition.   | String | optional | "2018" |
 | <a id="rust_toolchain-dylib_ext"></a>dylib_ext |  The extension for dynamic libraries created from rustc.   | String | required |  |
 | <a id="rust_toolchain-exec_triple"></a>exec_triple |  The platform triple for the toolchains execution environment. For more details see: https://docs.bazel.build/versions/master/skylark/rules.html#configurations   | String | required |  |
-| <a id="rust_toolchain-llvm_tools"></a>llvm_tools |  LLVM tools that are shipped with the Rust toolchain.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
+| <a id="rust_toolchain-llvm_tools"></a>llvm_tools |  LLVM tools that are shipped with the Rust toolchain.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "0", "opt": "3"} |
 | <a id="rust_toolchain-os"></a>os |  The operating system for the current toolchain   | String | required |  |
 | <a id="rust_toolchain-rust_doc"></a>rust_doc |  The location of the <code>rustdoc</code> binary. Can be a direct source or a filegroup containing one item.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |

rust/toolchain.bzl

@@ -533,7 +533,7 @@ rust_toolchain = rule(
             ),
             mandatory = True,
         ),
-        "llvm_tools": attr.label_list(
+        "llvm_tools": attr.label(
             doc = "LLVM tools that are shipped with the Rust toolchain.",
             allow_files = True,
         ),