[LSan] Parse common flags from LSAN_OPTIONS even if LSan is combined with

another sanitizer.

A user may run both LSan and LSan+ASan. It is weird to pass path to leak
suppression file (or other common sanitizer flags, like "verbosity") in
"LSAN_OPTIONS" in the first case and in "ASAN_OPTIONS" in the second case.

llvm-svn: 215949