[asan] enable LeakSanitizer (LSan) by default in asan. This only affects Linux x86_64. LSan has been used in various projects for more than half a year and we now consider it quite stable to be on by default.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@206896 91177308-0d34-0410-b5e6-96231b3b80d8