fix(diagnostic): deepcopy diagnostics before clamping line numbers

The current 'clamp_line_numbers' implementation modifies diagnostics in
place, which can have adverse downstream side effects. Before clamping
line numbers, make a copy of the diagnostic. This commit also merges the
'clamp_line_numbers' method into a new 'get_diagnostics' local function
which also implements the more general "get" method. The public
'vim.diagnostic.get()' API now just uses this function (without
clamping). This has the added benefit that other internal API functions
that need to use get() no longer have to go through vim.validate.

Finally, reorganize the source code a bit by grouping all of the data
structures together near the top of the file.

runtime/lua/vim/lsp/diagnostic.lua

@@ -717,5 +717,3 @@ end
 -- }}}
 
 return M
-
--- vim: fdm=marker