Add __sanitizer_get_allocated_begin API and implementations

This function will return the start of the allocation, if given a pointer that lies within an allocation. Otherwise, it returns NULL.

It will be useful for detecting dynamic TLS allocations in glibc >=2.25, which
uses malloc (see https://github.com/google/sanitizers/issues/1409#issuecomment-1214244142).

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D147005