Capture the computed shadow begin/end values at the point where the
shadow is first created and reuse those values on reset. Introduce new
windows-specific function "ZeroMmapFixedRegion" for zeroing out an
address space region previously returned by one of the MmapFixed*
routines; call this function (on windows) from DoResetImpl
tsan_rtl.cpp instead of MmapFixedSuperNoReserve.
See https://github.com/golang/go/issues/53539#issuecomment-1168778740
for context; intended to help with updating the syso for Go's
windows/amd64 race detector.
Differential Revision: https://reviews.llvm.org/D128909
b6374437af