Detect missing else in let statement

[bb1yd]

relevant issue:#135857

I add the help message in rustc_resolve. It now will display something like this:

error: expected identifier, found keyword `return`
 --> ./test.rs:8:24
  |
8 |      let Some(a) = bar{return;};
  |                    --- ^^^^^^ expected identifier, found keyword
  |                    |
  |                    while parsing this struct
  |
help: escape `return` to use it as an identifier
  |
8 |      let Some(a) = bar{r#return;};
  |                        ++

error[E0574]: expected struct, variant or union type, found local variable `bar`
 --> ./test.rs:8:20
  |
8 |      let Some(a) = bar{return;};
  |                    ^^^ not a struct, variant or union type
  |
help: try adding `else` here:
  |
8 |      let Some(a) = bar else {return;};
  |                        ++++

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0574`.

r? estebank

[bb1yd]

@bors squash msg="fix issue-156949"

[rust-bors]

🔨 4 commits were squashed into 6bbafec.