Disallow EOF in `nonterminal_may_begin_with()`

Until now, `:tt`, `:item`, and `:stmt` metavars could be
attempted against `token::Eof`. *HOWEVER*, when a `token::Eof` is
processed, `parse_tt_inner()` ignores `bb_mps`, not even checking
for ambiguity. It's as if those `bb_mps` never existed; as if
`nonterminal_may_begin_with()` returned `false` for `Eof`. This commit
makes that behavior more explicit.

This does not change user visible behavior.

`tests/ui/macros` pass.