Prior to supporting MemorySanitizer instrumentation of Reactor routines,
false positives in C++ code were avoided by marking all memory that was
written to by the Reactor routines as unpoisoned.

Now this can be disabled to avoid false negatives, i.e. uses of
uninitialized data either in the Reactor routine or originating from it.

REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION controls whether we get
the new behavior or the old unpoisoning.

Bug: b/155148722
Change-Id: Ia35e3cdc1a60ba44045869884c64e9875c030291
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49889
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>