Necessary braces in the left hand side of `:` are removed

```
$ cat haskell/repro.hs
foo c = case c of {_->c} : []

$ ormolu haskell/repro.hs
haskell/repro.hs
  AST of input and AST of formatted code differ.
    at haskell/repro.hs:1:9-29
  Please, consider reporting the bug.

$ ormolu --version
ormolu 0.3.0.0 HEAD 06b767c8b70e60321a4debc4599127446f958e5b
using ghc-lib-parser 9.0.1.20210324
```