IMPALA-2778: use correct llvm-symbolizer for ASAN
IMPALA-2778: use correct llvm-symbolizer for ASAN

Previously when ASAN tried to print a stack it used the llvm-symbolizer
executable in PATH. This may be the wrong version, e.g. LLVM 3.3 rather
than LLVM 3.7. On some builds this meant that we didn't get a useful
stack trace from ASAN.

Tested the change manually by adding a use-after-free to a backend test,
replicating the problem, then confirming that the problem was fixed
after sourcing the modified impala-config.sh

Change-Id: I00ab390485de12c6635ae446a55a49140fc05b37
Reviewed-on: http://gerrit.cloudera.org:8080/1649
Reviewed-by: Martin Grund <2546548d516fa64c411fde8242af35f3f80ad31f@cloudera.com>
Tested-by: Internal Jenkins

ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC