[sanitizer] Support dynamic premapped R/W range in primary allocator.

The main use case for this change is HWASan aliasing mode, which premaps
the alias space adjacent to the dynamic shadow.  With this change, the
primary allocator can allocate from the alias space instead of a
separate region.

Reviewed By: vitalybuka, eugenis

Differential Revision: https://reviews.llvm.org/D98293