[lldb] Do not deallocate memory after exec

After an exec has occured, resources used to manage the state of a
Process are cleaned up. One such resource is the AllocatedMemoryCache
which keeps track of memory allocations made in the process for things
like expression evaluation. After an exec is performed, the allocated
memory regions in the process are gone, so it does not make sense to try
to deallocate those regions.

rdar://103188106

Differential Revision: https://reviews.llvm.org/D140249