[DSLX] Factor out diagnostic routines, make test, add one diagnostic routine.

[cdleary]

Now we give a more helpful explanation if you put a trailing semicolon in one block within an if/else ladder that gives an indication that users should see if that was intended. I observed it was common for folks to type an accidental trailing semi and then try to decipher why they were getting the error particularly if the consequent/alternate blocks were large.

• Most of this change by volume is refactoring things into the diagnostics/ dir
• But it also adds xls/dslx/diagnostics/maybe_explain_error_test.cc
• It exposes two internal routines (in an internal namespace) to enable that test -- previously all diagnostic tests had to be done at the language level
• Conditional::GatherBlocks is added to collect all the blocks participating in an if/else ladder
• AstNode::SetEncosing() / GetEnclosing() is added to supplement the parent() relation -- because conditional nodes nest we want a direct (and easy) link to the "primordial" conditional in the ladder, and if you traversed parent() links you would have trouble telling when to stop (e.g. consider two nested conditional ladders) -- parent() relation is already completely defined as "tell me the node who claims me via GetChildren()" so we needed something new or a complicated traversal criterion