Per the DAP spec for SetBreakpoints [1], the way to clear breakpoints is: `To clear all breakpoint for a source, specify an empty array.`
However, leaving the breakpoints field unset is also a well formed request (note the `breakpoints?:` in the `SetBreakpointsArguments` definition). If it's unset, we have a couple choices:
1. Crash (current behavior)
2. Clear breakpoints
3. Return an error response that the breakpoints field is missing.
I propose we do (2) instead of (1), and treat an unset breakpoints field the same as an empty breakpoints field.
[1] https://microsoft.github.io/debug-adapter-protocol/specification#Requests_SetBreakpoints
Reviewed By: wallace, labath
Differential Revision: https://reviews.llvm.org/D88513
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