`=>` to `=` typo in match arm results in lots of parse errors

[estebank]

A typo of => to = in a match arm currently results in a lot of cascading errors.

error: expected one of `!`, `(`, `...`, `..=`, `..`, `::`, `=>`, `if`, `{`, or `|`, found `=`
   --> compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:244:39
    |
244 |             SelectionError::Ambiguous = {
    |                                       ^ expected one of 10 possible tokens

We should gracefully handle some cases like > and = instead of => if followed by a { in a match arm.

[SkiFire13]

-> is also a common case that could be handled better