Bug 18760 – theAllocator should hold a reference to the allocator that it uses

Currently, theAllocator does not hold a reference to the underlying process allocator that does the work. Otherwise, changing the processAllocator will result in segfault/UB on behalf of theAllocator. The following snippet better describes the issue, causing a segfault ``` import std.experimental.allocator.mallocator : Mallocator; auto a = sharedAllocatorObject(Mallocator.instance); auto buf = theAllocator.allocate(42); processAllocator = a; theAllocator.deallocate(buf); ```

Commit pushed to master at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/4126f928270570c9149a6412acde16f16506fd39 Merge pull request #6449 from edi33416/fix_thread_alloc Fix Issue 18760 - theAllocator dangling reference bug