Unbreak ASan runtime in the simulators.

Summary:
861b69fa (rdar://problem/58789439) while
fixing symbolization for TSan completely broke ASan's runtime for the
simulators.

The problem with the previous patch is that the memory passed to
`putenv()` was poisoned and when passed to `putenv()` it tripped
an interceptor for `strchr()` which saw the memory was poisoned and
raised an ASan issue.

The memory was poisoned because `AtosSymbolizerProcess` objects
are created using ASan's internal allocator. Memory from this
allocator gets poisoned with `kAsanInternalHeapMagic`.

To workaround this, this patch makes the memory for the environment
variable entry a global variable that isn't poisoned.

This pass also adds a `DCHECK(getenv(K_ATOS_ENV_VAR))` because the
following DCHECK would crash because `internal_strcmp()` doesn't
work on nullptr.

rdar://problem/62067724

Reviewers: kubamracek, yln

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D78525