[LSan] Make parent tool responsible for initializing LSan flags.

Summary:
LSan can be combined with a parent tool (for now it's only ASan).
Also, we allow LSAN_OPTIONS to override certain common flags. It means
we have to parse LSAN_OPTIONS early enough, before the rest of the
parent tool (including chunks of sanitizer_common) is initialized.

In future, we can use the same approach for UBSan, after we embed it
into ASan runtime in a similar way.

Test Plan: regression test suite

Reviewers: earthdok, eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7577

llvm-svn: 229519