[sanitizer] introduce LargeMmapAllocator::GetBlockBeginFastSingleThreaded, required for LeakSanitizer to work faster. Also fix lint.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182917 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed