Update msan symbolizer path

To account for the newer version of LLVM on Ubuntu 18.04.

Author: nikic

azure/test.yml

@@ -12,7 +12,7 @@ steps:
       export TEST_PHP_JUNIT=junit.xml
       export REPORT_EXIT_STATUS=no
       export SKIP_IO_CAPTURE_TESTS=1
-      export MSAN_SYMBOLIZER_PATH=/usr/lib/llvm-3.8/bin/llvm-symbolizer
+      export MSAN_SYMBOLIZER_PATH=/usr/lib/llvm-6.0/bin/llvm-symbolizer
       rm -rf junit.xml | true
       php run-tests.php -P -q \
           -j$(/usr/bin/nproc) \