NativeProcessProtocol: Simplify breakpoint setting code

Summary:
A fairly simple operation as setting a breakpoint (writing a breakpoint
opcode) at a given address was going through three classes:
NativeProcessProtocol which called NativeBreakpointList, which then
called SoftwareBrekpoint, only to end up again in NativeProcessProtocol
to do the actual writing itself. This is unnecessarily complex and can
be simplified by moving all of the logic into NativeProcessProtocol
class itself, removing a lot of boilerplate.

One of the reeasons for this complexity was that (it seems)
NativeBreakpointList class was meant to hold both software and hardware
breakpoints. However, that never materialized, and hardware breakpoints
are stored in a separate map holding only hardware breakpoints.
Essentially, this patch makes software breakpoints follow that approach
by replacing the heavy SoftwareBraekpoint with a light struct of the
same name, which holds only the data necessary to describe one
breakpoint. The rest of the logic is in the main class. As, at the
lldb-server level, handling software and hardware breakpoints is very
different, this seems like a reasonable state of things.

Reviewers: krytarowski, zturner, clayborg

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D52941

llvm-svn: 346093