[Sanitizer] Remove CombinedAllocator::Allocate's 'cleared' parameter

Summary:
CombinedAllocator::Allocate cleared parameter is not used anywhere and
seem to be obsolete.

Reviewers: eugenis

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34289

llvm-svn: 305590