Refactor momentary breakpoints, eliminate set_raw_breakpoint{,_without_location}

Refactor momentary breakpoints, eliminate set_raw_breakpoint{,_without_location}

This commit makes set_momentary_breakpoint allocate the breakpoint
type without relying on set_raw_breakpoint, and similarly,
momentary_breakpoint_from_master not rely on
set_raw_breakpoint_without_location.  This will let us convert
init_raw_breakpoint to a ctor in a following patch.

The comment about set_raw_breakpoint being used in gdbtk sources is
stale.  gdbtk no longer uses it.

Change-Id: Ibbf77731e4b22e18ccebc1b5799bbec0aff28c8a