Export of internal Abseil changes

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93dd18f415e54697965c56760612b4ff6c9d15d2 by Derek Mauro <dmauro@google.com>:

Implement Bazel's --distdir flag avoid a dependency on GitHub being up.
Move the Docker container identifiers to a common file.

PiperOrigin-RevId: 302045549

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7b462d31139e3153462cd20ae63b161ac749e839 by Derek Mauro <dmauro@google.com>:

Fix an unused variable warning in optional.h

PiperOrigin-RevId: 301810782

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6b533ba9170eed065aec69e48ebab3279fcb3e09 by Abseil Team <absl-team@google.com>:

Disable ASan and MSan instrumentation for AArch64 stack unwind functions. They read random locations on the stack and might step on sanitizer's redzones.

PiperOrigin-RevId: 301674204
GitOrigin-RevId: 93dd18f415e54697965c56760612b4ff6c9d15d2
Change-Id: I2d84d66b054f90b851fb3a968679f112dfe84636

ci/linux_gcc-4.9_libstdcxx_bazel.sh

@@ -36,7 +36,8 @@ if [ -z ${EXCEPTIONS_MODE:-} ]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-4.9:20191018"
+source "${ABSEIL_ROOT}/ci/linux_docker_containers.sh"
+readonly DOCKER_CONTAINER=${LINUX_GCC_49_CONTAINER}
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -50,6 +51,14 @@ if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
   BAZEL_EXTRA_ARGS="--remote_http_cache=https://storage.googleapis.com/absl-bazel-remote-cache/${container_key} --google_credentials=/keystore/73103_absl-bazel-remote-cache ${BAZEL_EXTRA_ARGS:-}"
 fi
 
+# Avoid depending on external sites like GitHub by checking --distdir for
+# external dependencies first.
+# https://docs.bazel.build/versions/master/guide.html#distdir
+if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+  DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
+  BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
+fi
+
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do