[BreakpointList] Simplify/modernize BreakpointList (NFC)

I was looking at the code in BreakpointList.cpp and found it deserved a
quick cleanup.

  -  Use std::vector instead of a std::list.
  -  Extract duplicate code for notifying.
  -  Remove code duplication when returning a const value.
  -  Use range-based for loop.
  -  Use early return in loops.

Differential revision: https://reviews.llvm.org/D56425

llvm-svn: 350659