Ban globals in default args if ctx lacks AccessGlobals [1/2]

Ban globals in default args if ctx lacks AccessGlobals [1/2]

Summary:
Fixes the unsoundness by not introducing the AccessGlobals out of thin air
when a `function`'s context would not otherwise give this capability, e.g.:

  function f(int $x = AnotherClass::$staticInt)[]

should _not_ be allowed (at the very least when called from a pure context),
but the typechecker previously allowed it.  Therefore, check if `AccessGlobals`
is present before type-checking the parameter list and the matching arguments,
and _only then_ reintroduce the capability the `AcceessGlobals`. E.g.,

  function g(int $x = Another::$staticInt)[defaults]

is sound to allow because `AccessGlobals` is contained in `defaults`.

 ---

However, this is still too conservative, since the above startegy rejects:

  function h(int $x = readonly Another::$staticInt)[read_globals]

The extra precision comes at the cost of doing additional subtyping check:

  ReadGlobals <: FUN_CTX

and if so, introducing `ReadGlobals` instead of `AccessGlobals` when
type-checking argument binding for a function call.  As an optimization,
we could check that the default argument is marked `readonly` and then
only do the latter subtyping check but not the former.
This optimized approach will be considered as a separate change.

Reviewed By: vassilmladenov

Differential Revision: D30547750

fbshipit-source-id: 8b3952e8d8db612684bb3f11bc5cf479301ef0d4