Change Target::FindBreakpointsByName to return Expected<vector>

Summary:
Using a BreakpointList corrupts the breakpoints' IDs because
BreakpointList::Add sets the ID, so use a vector instead, and
update the signature to return the vector wrapped in an
llvm::Expected which can propagate any error from the inner
call to StringIsBreakpointName.

Note that, despite the similar name, SBTarget::FindBreakpointsByName
doesn't suffer the same problem, because it uses a SBBreakpointList,
which is more like a BreakpointIDList than a BreakpointList under the
covers.

Add a check to TestBreakpointNames that, without this fix, notices the
ID getting mutated and fails.

Reviewers: jingham, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70907