[lldb] Don't use hardware index to determine whether a breakpoint site is hardware

Most process plugins (if not all) don't set hardware index for breakpoints. They even
are not able to determine this index.

This patch makes StoppointLocation::IsHardware pure virtual and lets BreakpointSite
override it using more accurate BreakpointSite::Type.

It also adds assertions to be sure that a breakpoint site is hardware when this is required.

Differential Revision: https://reviews.llvm.org/D84257