upgrade rust to 1.37 (#5611)

ci/docker-rust/Dockerfile

@@ -1,6 +1,6 @@
 # Note: when the rust version is changed also modify
 # ci/rust-version.sh to pick up the new image tag
-FROM rust:1.36.0
+FROM rust:1.37.0
 
 # Add Google Protocol Buffers for Libra's metrics library.
 ENV PROTOC_VERSION 3.8.0
@@ -10,9 +10,7 @@ RUN set -x \
  && apt update \
  && apt-get install apt-transport-https \
  && echo deb https://apt.buildkite.com/buildkite-agent stable main > /etc/apt/sources.list.d/buildkite-agent.list \
- && echo deb http://apt.llvm.org/stretch/ llvm-toolchain-stretch-7 main > /etc/apt/sources.list.d/llvm.list \
  && apt-key adv --no-tty --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 32A37959C2FA5C3C99EFBC32A79206696452D198 \
- && wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - \
  && apt update \
  && apt install -y \
       buildkite-agent \
@@ -20,7 +18,6 @@ RUN set -x \
       cmake \
       lcov \
       libclang-common-7-dev \
-      llvm-7 \
       mscgen \
       rsync \
       sudo \