The recommended way forward is to use the SyntaxClassifier on the Swift
side.
By removing the C++ SyntaxClassifier, we can also eliminate the
-force-libsyntax-based-processing option that was used to bootstrap
incremental parsing and would generate the syntax map from a syntax
tree.
We still need to adjust the affected range to the line boundaries and return all
tokens on the line when there are no new tokens, as the client will clear all
tokens on that line in its copy of the syntax map leaving the other tokens
unhighlighted. We also need to extend the affected range to include the ranges
of the mismatched tokens from the previous syntaxmap, so their highlighting will
be cleared.
Also add more comments to better document the new syntax map structure and
behaviour.
