sanitizers: consistently check result of MmapFixedNoReserve

MmapFixedNoReserve does not terminate process on failure.
Failure to check its result and die will always lead to harder
to debug crashes later in execution. This was observed in Go
processes due to some address space conflicts.

Consistently check result of MmapFixedNoReserve.
While we are here also add warn_unused_result attribute
to prevent such bugs in future and change return type to bool
as that's what all callers want.

Reviewed in https://reviews.llvm.org/D49367

llvm-svn: 337531