[sanitizer] Do not use the alignment-rounded-up size when using the secondary

Summary:
The combined allocator rounds up the requested size with regard to the
alignment, which makes sense when being serviced by the primary as it comes
with alignment guarantees, but not with the secondary. For the rare case of
large alignments, it wastes memory, and entices unnecessarily large fields for
the Scudo header. With this patch, we pass the non-alignement-rounded-up size
to the secondary, and adapt the Scudo code for this change.

Reviewers: alekseyshl, kcc

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D27428

llvm-svn: 289088