ICE when Unicode character follows incomplete expression

[KamilaBorowska]

This is a variation on #68091 that despite similarity I think is completely different and probably should be tracked separately. Regression occured in Rust 1.34.

macro_rules! x {
    ($($c:tt)*) => {
        $($c)ö*
    };
}

fn main() {
    x!(!);
}

Stacktrace:

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32', src/libsyntax_pos/source_map.rs:836:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1057
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:195
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:215
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  12: std::panicking::begin_panic
  13: syntax_pos::source_map::SourceMap::bytepos_to_file_charpos
  14: syntax_pos::source_map::SourceMap::lookup_char_pos
  15: rustc_errors::emitter::EmitterWriter::get_multispan_max_line_num
  16: <rustc_errors::emitter::EmitterWriter as rustc_errors::emitter::Emitter>::emit_diagnostic
  17: <rustc_errors::json::JsonEmitter as rustc_errors::emitter::Emitter>::emit_diagnostic
  18: rustc_errors::HandlerInner::emit_diagnostic
  19: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit
  20: syntax_expand::mbe::macro_rules::ParserAnyMacro::make
  21: syntax_expand::expand::<impl syntax_expand::base::MacResult for syntax_expand::mbe::macro_rules::ParserAnyMacro>::make_stmts
  22: syntax_expand::expand::AstFragmentKind::make_from
  23: syntax_expand::expand::MacroExpander::fully_expand_fragment
  24: syntax_expand::expand::MacroExpander::expand_crate
  25: rustc_interface::passes::configure_and_expand_inner::{{closure}}
  26: rustc::util::common::time
  27: rustc_interface::passes::configure_and_expand_inner
  28: rustc_interface::passes::configure_and_expand::{{closure}}
  29: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
  30: rustc_interface::passes::configure_and_expand
  31: rustc_interface::queries::Queries::expansion
  32: rustc_interface::interface::run_compiler_in_existing_thread_pool
  33: scoped_tls::ScopedKey<T>::set
  34: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (da3629b05 2019-12-29) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `playground`.

To learn more, run the command again with --verbose.