Update Cargo submodule

Bring in some fixes for `cargo fix` notably

src/tools/rustc-workspace-hack/Cargo.toml

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+[package]
+name = "rustc-workspace-hack"
+version = "1.0.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+license = 'MIT/Apache-2.0'
+description = """
+Hack for the compiler's own build system
+"""
+
+[lib]
+path = "lib.rs"
+
+# For documentation about what this is and why in the world these dependencies
+# are appearing, see `README.md`.
+
+[build-dependencies]
+# Currently Cargo/RLS depend on `failure` which depends on `synstructure` which
+# enables this feature. Clippy, however, does not depend on anything that
+# enables this feature. Enable it unconditionally.
+syn = { version = "0.14", features = ['extra-traits'] }
+
+[target.'cfg(windows)'.dependencies.winapi]
+version = "0.3"
+features = [
+  "profileapi",
+  "memoryapi",
+  "minschannel",
+  "securitybaseapi",
+  "jobapi2",
+  "schannel",
+  "sysinfoapi",
+  "jobapi",
+  "synchapi",
+  "wincrypt",
+  "winbase",
+  "minwinbase",
+  "ntsecapi",
+  "basetsd",
+  "ntstatus",
+  "psapi",
+  "timezoneapi",
+  "lmcons",
+  "wincon",
+]

src/tools/rustc-workspace-hack/README.md

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+# `rustc-workspace-hack`
+
+This crate is a bit of a hack to make workspaces in rustc work a bit better.
+The rationale for this existence is a bit subtle, but the general idea is that
+we want commands like `./x.py build src/tools/{rls,clippy,cargo}` to share as
+many dependencies as possible.
+
+Each invocation is a different invocation of Cargo, however. Each time Cargo
+runs a build it will re-resolve the dependency graph, notably selecting
+different features sometimes for each build.
+
+For example, let's say there's a very deep dependency like `num-traits` in each
+of these builds. For Cargo the `num-traits`'s `default` feature is turned off.
+In RLS, however, the `default` feature is turned. This means that building Cargo
+and then the RLS will actually build Cargo twice (as a transitive dependency
+changed). This is bad!
+
+The goal of this crate is to solve this problem and ensure that the resolved
+dependency graph for all of these tools is the same in the various subsets of
+each tool, notably enabling the same features of transitive dependencies.
+
+All tools vendored here depend on the `rustc-workspace-hack` crate on crates.io.
+When on crates.io this crate is an empty crate that is just a noop. We override
+it, however, in this workspace to this crate here, which means we can control
+crates in the dependency graph for each of these tools.

src/tools/rustc-workspace-hack/lib.rs

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+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// intentionally left blank