Bump system LLVM version for travis builds to 7

.travis.yml

@@ -16,7 +16,7 @@ matrix:
   fast_finish: true
   include:
     # Images used in testing PR and try-build should be run first.
-    - env: IMAGE=x86_64-gnu-llvm-5.0 RUST_BACKTRACE=1
+    - env: IMAGE=x86_64-gnu-llvm-7 RUST_BACKTRACE=1
       if: type = pull_request OR branch = auto
 
     - env: IMAGE=dist-x86_64-linux DEPLOY=1

src/ci/docker/x86_64-gnu-llvm-5.0/Dockerfile → src/ci/docker/x86_64-gnu-llvm-7/Dockerfile

@@ -11,17 +11,20 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   cmake \
   sudo \
   gdb \
-  llvm-5.0-tools \
   libedit-dev \
   zlib1g-dev \
   xz-utils
 
+RUN  curl https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - && \
+  echo deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main > /etc/apt/sources.list.d/llvm.list && \
+  apt-get update && apt-get install -y --no-install-recommends llvm-7-tools
+
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
 # using llvm-link-shared due to libffi issues -- see #34486
 ENV RUST_CONFIGURE_ARGS \
       --build=x86_64-unknown-linux-gnu \
-      --llvm-root=/usr/lib/llvm-5.0 \
+      --llvm-root=/usr/lib/llvm-7 \
       --enable-llvm-link-shared
 ENV RUST_CHECK_TARGET check