Add the ability to process save-core stack-memory-only corefiles

Add a field to the qMemoryRegionInfo packet where the remote stub
can describe the type of memory -- heap, stack.  Keep track of
memory regions that are stack memory in lldb.  Add a new "--style
stack" to process save-core to request that only stack memory be
included in the corefile.

Differential Revision: https://reviews.llvm.org/D107625