Change last-ditch magic address in IRMemoryMap::FindSpace

Change last-ditch magic address in IRMemoryMap::FindSpace

When we cannot allocate memory in the inferior process, the IR
interpreter's IRMemoryMap::FindSpace will create an lldb local
buffer and assign it an address range in the inferior address
space.  When the interpreter sees an address in that range, it
will read/write from the local buffer instead of the target.  If
this magic address overlaps with actual data in the target, the
target cannot be accessed through expressions.

Instead of using a high memory address that is validly addressable,
this patch uses an address that cannot be accessed on 64-bit systems
that don't actually use all 64 bits of the virtual address.

Differential Revision: https://reviews.llvm.org/D137682
rdar://96248287