ICE with Unicode character following `if` in `if` conditional without condition

[KamilaBorowska]

Inspired by #68000. Nobody will write code like this ever, but it's still an ICE. This regression was introduced in Rust 1.25, this worked in Rust 1.24.

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

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

Stacktrace on playground:

   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/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-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: rustc_parse::parser::expr::<impl rustc_parse::parser::Parser>::parse_if_expr
  21: rustc_parse::parser::expr::<impl rustc_parse::parser::Parser>::parse_bottom_expr
  22: rustc_parse::parser::expr::<impl rustc_parse::parser::Parser>::parse_prefix_expr
  23: rustc_parse::parser::expr::<impl rustc_parse::parser::Parser>::parse_assoc_expr_with
  24: rustc_parse::parser::stmt::<impl rustc_parse::parser::Parser>::parse_stmt_without_recovery
  25: rustc_parse::parser::stmt::<impl rustc_parse::parser::Parser>::parse_full_stmt
  26: syntax_expand::expand::parse_ast_fragment
  27: syntax_expand::mbe::macro_rules::ParserAnyMacro::make
  28: syntax_expand::expand::<impl syntax_expand::base::MacResult for syntax_expand::mbe::macro_rules::ParserAnyMacro>::make_stmts
  29: syntax_expand::expand::AstFragmentKind::make_from
  30: syntax_expand::expand::MacroExpander::fully_expand_fragment
  31: syntax_expand::expand::MacroExpander::expand_crate
  32: rustc_interface::passes::configure_and_expand_inner::{{closure}}
  33: rustc::util::common::time
  34: rustc_interface::passes::configure_and_expand_inner
  35: rustc_interface::passes::configure_and_expand::{{closure}}
  36: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
  37: rustc_interface::passes::configure_and_expand
  38: rustc_interface::queries::Queries::expansion
  39: rustc_interface::interface::run_compiler_in_existing_thread_pool
  40: scoped_tls::ScopedKey<T>::set
  41: 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.