This commit adds support for column breakpoints to lldb-dap.
To do so, support for the `breakpointLocations` request was
added. To find all available breakpoint positions, we iterate over
the line table.
The `setBreakpoints` request already forwarded the column correctly to
`SBTarget::BreakpointCreateByLocation`. However, `SourceBreakpointMap`
did not keep track of multiple breakpoints in the same line. To do so,
the `SourceBreakpointMap` is now indexed by line+column instead of by
line only.
See http://jonasdevlieghere.com/post/lldb-column-breakpoints/ for a
high-level introduction to column breakpoints.
4f48a81a62