lexer: show correct span on lexical errors

Previously, the lexer calling `rdr.fatal(...)` would report the span of
the last complete token, instead of a span within the erroneous token
(besides one span fixed in 1ac90bb).

This commit adds a wrapper around `rdr.fatal(...)` that sets the span
explicilty, so that all fatal errors in `libsyntax/parse/lexer.rs` now
report the offending code more precisely. A number of tests try to
verify that, though the `compile-fail` testing setup can only check that
the spans are on the right lines, and the "unterminated string/block
comment" errors can't have the line marked at all, so that's incomplete.

Closes #9149.