[clangd] Handle an expanded token range that ends in the `eof` token in...

[clangd] Handle an expanded token range that ends in the `eof` token in TokenBuffer::spelledForExpanded() (#78092)

Such ranges can legitimately arise in the case of invalid code, such as
a declaration missing an ending brace.

Fixes https://github.com/clangd/clangd/issues/1559