[lldb] Add AllocateMemory/DeallocateMemory to the SBProcess API

This change adds AllocateMemory and DeallocateMemory methods to the SBProcess
API, so that clients can allocate and deallocate memory blocks within the
process being debugged (for storing JIT-compiled code or other uses).

(I am developing a debugger + REPL using the API; it will need to store
JIT-compiled code within the target.)

Reviewed By: clayborg, jingham

Differential Revision: https://reviews.llvm.org/D105389