[lsan][Darwin] Scan libdispatch and Foundation memory regions

libdispatch uses its own heap (_dispatch_main_heap) for some allocations, including the dispatch_continuation_t that holds a dispatch source's event handler.
Objective-C block trampolines (creating methods at runtime with a block as the implementations) use the VM_MEMORY_FOUNDATION region (see https://github.com/apple-oss-distributions/objc4/blob/8701d5672d3fd3cd817aeb84db1077aafe1a1604/runtime/objc-block-trampolines.mm#L371).

This change scans both regions to fix false positives. See tests for details; unfortunately I was unable to reduce the trampoline example with imp_implementationWithBlock on a new class, so I'm resorting to something close to the bug as seen in the wild.

Differential Revision: https://reviews.llvm.org/D129385