Bump the minimum LLVM to 3.9

Old LLVM bugs are reportedly cropping up harder, but 3.9 seems to be OK.

Fixes #45277.

.travis.yml

@@ -12,7 +12,7 @@ matrix:
   fast_finish: true
   include:
     # Images used in testing PR and try-build should be run first.
-    - env: IMAGE=x86_64-gnu-llvm-3.7 RUST_BACKTRACE=1
+    - env: IMAGE=x86_64-gnu-llvm-3.9 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-3.7/Dockerfile → src/ci/docker/x86_64-gnu-llvm-3.9/Dockerfile

@@ -11,15 +11,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   cmake \
   sudo \
   gdb \
-  llvm-3.7-tools \
+  llvm-3.9-tools \
   libedit-dev \
   zlib1g-dev \
   xz-utils
 
 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-3.7
+      --llvm-root=/usr/lib/llvm-3.9 \
+      --enable-llvm-link-shared
 ENV RUST_CHECK_TARGET check