[clangd] Support multiline semantic tokens

[clangd] Support multiline semantic tokens

Per LSP, multiline tokens should be handled as if they end at the end
of the line starting the token (there's also a capability to enable them, but
that's an adventure for a different day).

Fixes https://github.com/clangd/clangd/issues/1145

Differential Revision: https://reviews.llvm.org/D127856