Bump XSAN build to bionic and add status message

.travis.yml

@@ -98,7 +98,6 @@ matrix:
 #    env: BUILD_TYPE=Debug XSAN=undefined PATH="/usr/local/opt/ccache/libexec:$PATH" CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++
   - name: "memory sanitizer"
     os: linux
-    dist: xenial
     compiler: clang
     env: BUILD_TYPE=Debug XSAN=memory
     addons: *core_apt_addons

CMakeLists.txt

@@ -105,6 +105,7 @@ endif()
 if(XSAN)
   set(DEBUG_FLAGS ${DEBUG_FLAGS} "-fsanitize=${XSAN}" -fno-omit-frame-pointer)
   set(OPTIMIZE_FLAGS "-O0")
+  message(STATUS "Doing a ${XSAN} sanitizer build")
 endif(XSAN)
 
 if(CMAKE_BUILD_TYPE MATCHES "[Dd][Ee][Bb][Uu][Gg]")