Note [RequiredTypeArguments and the T2T mapping]

The "T2T-Mapping" section of GHC Proposal #281 introduces a term-to-type transformation
that comes into play when we typecheck function applications to required type arguments.
Say we have a function that expects a required type argument, vfun :: forall a -> ...
then it is possible to call it as follows:

  vfun (Maybe Int)

The Maybe Int argument is parsed and renamed as a term. There is no syntactic marker
to tell GHC that it is actually a type argument.  We only discover this by the time
we get to type checking, where we know that f's type has a visible forall at the front,
so we are expecting a type argument. More precisely, this happens in tcVDQ in GHC/Tc/Gen/App.hs:

  tcVDQ :: ConcreteTyVars              -- See Note [Representation-polymorphism checking built-ins]
        -> (ForAllTyBinder, TcType)    -- Function type
        -> LHsExpr GhcRn               -- Argument type
        -> TcM (TcType, TcType)

What we want is a type to instantiate the forall-bound variable. But what we have is an HsExpr,
and we need to convert it to an HsType in order to reuse the same code paths as we use for
checking f @ty (see tc_inst_forall_arg).

  f (Maybe Int)
  ^^^^^^^^^
  parsed and renamed as:   HsApp   (HsVar   "Maybe") (HsVar   "Int")  ::  HsExpr GhcRn
  must be converted to:    HsTyApp (HsTyVar "Maybe") (HsTyVar "Int")  ::  HsType GhcRn

We do this using a helper function:

  expr_to_type :: LHsExpr GhcRn -> TcM (LHsWcType GhcRn)

This conversion is in the TcM monad because
* It can fail, if the expression is not convertible to a type.
      vfun [x | x <- xs]     Can't convert list comprehension to a type
      vfun (\x -> x)         Can't convert a lambda to a type
* It needs to check for LangExt.NamedWildCards to generate an appropriate
  error message for HsHole (HsVar ...).
     vfun _a    Not in scope: ‘_a’
                   (NamedWildCards disabled)
     vfun _a    Illegal named wildcard in a required type argument: ‘_a’
                   (NamedWildCards enabled)