[LeakSanitizer] Capture calling thread SP early to avoid false negatives.

As shown in https://github.com/llvm/llvm-project/issues/42932 dead
pointers might be overlapped by a new stack frame inside CheckForLeaks,
which does not use bytes with pointers. This leads to false negatives.

It's not a full solution for the problem as it does not solve
"overlapping" new/old frames for frames below the CheckForLeaks and in
other threads. It should improve leaks found in direct callers of
__lsan_do_leak_check.

Differential Revision: https://reviews.llvm.org/D130237