[asan] Add fast_unwind_on_check flag.

Allows to specify the unwinder to use for CHECK failures. Previous behaviour
was to use the "fatal" unwinder.
As compiler-rt is built without frame pointers, only the slow unwinder
really makes sense here, and it is the default.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219677 91177308-0d34-0410-b5e6-96231b3b80d8