Handle fully enclosed multiline spans gracefully in emitter

[estebank]

Currently the solution to overlapping multiline spans is correct, but doesn't handle spans that are completely enclosed by others in any way, simply using span ordering and growing towards the left as needed. The following is a representative example of the problem:

error[E0308]: if and else have incompatible types
    --> src/libsyntax/parse/parser.rs:2640:33
     |
2636 |                                 let (span, msg) = if self.token == token::Token::Eof && self.is_subparser {
     |    _______________________________________________-
2637 |   |                                 (self.sess.source_map().next_point(self.span),
     |  _|_________________________________-
2638 | | |                                  "expected expression, found end of macro arguments")
     | |_|_____________________________________________________________________________________- expected because of this
2639 |   |                             } else {
2640 | / |                                 (self.span, format!(
2641 | | |                                     "expected expression, found {}",
2642 | | |                                     self.this_token_descr(),
2643 | | |                                 ))
     | |_|__________________________________^ expected &str, found struct `std::string::String`
2644 |   |                             };
     |   |_____________________________- if and else have incompatible types
     |
     = note: expected type `(syntax_pos::span_encoding::Span, &str)`
                found type `(syntax_pos::span_encoding::Span, std::string::String)`

Ideally, the emitter should handle this case by identifying that there are overlapping spans that are fully enclosed and special case the output to make it easier to read:

error[E0308]: if and else have incompatible types
    --> src/libsyntax/parse/parser.rs:2640:33
     |
2636 |                                let (span, msg) = if self.token == token::Token::Eof && self.is_subparser {
     |  ________________________________________________-
2637 | | /                                (self.sess.source_map().next_point(self.span),
2638 | | |                                 "expected expression, found end of macro arguments")
     | | |____________________________________________________________________________________- expected because of this
2639 | |                              } else {
2640 | | /                                (self.span, format!(
2641 | | |                                    "expected expression, found {}",
2642 | | |                                    self.this_token_descr(),
2643 | | |                                ))
     | | |_________________________________^ expected &str, found struct `std::string::String`
2644 | |                              };
     | |______________________________- if and else have incompatible types
     |
     = note: expected type `(syntax_pos::span_encoding::Span, &str)`
                found type `(syntax_pos::span_encoding::Span, std::string::String)`

[estebank]

The current output is now

  |
2 |         let _ = if true {
  |  _______________-
3 | |           (1,
  | |  _________-
4 | | |         2)
  | | |__________- expected because of this
5 | |       } else {
6 | | /         ("",
7 | | |         2)
  | | |__________^ expected integer, found `&str`
8 | |       };
  | |_______- `if` and `else` have incompatible types
  |
  = note: expected tuple `({integer}, {integer})`
             found tuple `(&str, {integer})`

The only thing left to do is make the first case properly do the / start span special case handling.