Reorganize TypeGuard code a bit

Summary:
I'm about to add support for `TypeIs` to Pyre, and a majority of the code
should be shared with `TypeGuard`.

There are a few places where the existing code isn't optimized for extending
the concept of type guards to be more general, either due to poor factoring
or names that aren't as clear as they could be. This is a simple commit
with no functional changes to clean some of these up in preparation for
`TypeIs` support.

Reviewed By: grievejia

Differential Revision: D59734070

fbshipit-source-id: c892c2b526fbbc5a1fc17aeb4f74d138c065e316

source/analysis/type.ml

@@ -4612,7 +4612,7 @@ let expression_contains_any expression =
   |> List.exists ~f:(Hashtbl.mem primitives_with_any_map)
 
 
-let type_inside_typeguard = function
+let inner_type_if_is_typeguard = function
   | Parametric
       {
         name = "typing.TypeGuard" | "typing_extensions.TypeGuard";

source/analysis/type.mli

@@ -564,7 +564,7 @@ val is_untyped : t -> bool
 
 val contains_variable : t -> bool
 
-val type_inside_typeguard : t -> t option
+val inner_type_if_is_typeguard : t -> t option
 
 val parameters : t -> Parameter.t list option
 

source/analysis/typeCheck.ml

@@ -4157,7 +4157,7 @@ module State (Context : Context) = struct
         { arguments = { Call.Argument.name = None; value = { Node.value = Name name; _ } } :: _; _ }
       when is_simple_name name -> (
         let { TypeInfo.Unit.annotation = callee_type; _ } = resolve_expression ~resolution test in
-        match Type.type_inside_typeguard callee_type with
+        match Type.inner_type_if_is_typeguard callee_type with
         | Some guard_type ->
             let resolution = refine_local ~name (TypeInfo.Unit.create_mutable guard_type) in
             Value resolution