Note [splitHsApps]

The key function
  splitHsApps :: HsExpr GhcRn -> (HsExpr GhcRn, HsExpr GhcRn, [HsExprArg 'TcpRn])
takes apart either an HsApp, or an infix OpApp, returning

* The "head" of the application, an expression that is often a variable;
  this is used for typechecking

* The "user head" or "error head" of the application, to be reported to the
  user in case of an error.  Example:
         (`op` e)
  expands (via ExpandedThingRn) to
         (rightSection op e)
  but we don't want to see 'rightSection' in error messages. So we keep the
  innermost un-expanded head as the "error head".

* A list of HsExprArg, the arguments