[asan]  Assign a low branch weight to ASan's slow path, patch by Jonas Wagner. This speeds up asan (at least on SPEC) by 1%-5% or more. Also fix lint in dfsan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216972 91177308-0d34-0410-b5e6-96231b3b80d8