Cursor info for a constructor would previously give the cursor info for
the containing type only. It now also adds cursor info for the
constructor itself in a "secondary_symbols" field.
Refactor `passCursorInfoForDecl` to use a single allocator rather than
keeping track of positions in a buffer and assigning everything at the
end of the function.
Refactor the various available refactoring gathering functions to take a
SmallVectorImpl and to not copy strings where they don't need to.
Resolves rdar://75385556
