fix callble typehint in interp mode

Somehow the JIT doesn't hit this codepath, but with interp mode the param is a string, but we think `callable` means an object of type callable.

I hate to hadd another top-level if statement to every call. Would a switch statement on the type be better?

hphp/runtime/vm/type_constraint.cpp

@@ -182,9 +182,14 @@ TypeConstraint::check(const TypedValue* tv, const Func* func) const {
     return true;
   }
 
-  return isObjectOrTypedef() && !isCallable()
-    ? checkTypedefNonObj(tv)
-    : equivDataTypes(m_type.m_dt, tv->m_type);
+  if (isObjectOrTypedef()) {
+    if (isCallable()) {
+      return f_is_callable(tvAsCVarRef(tv));
+    }
+    return checkTypedefNonObj(tv);
+  }
+
+  return equivDataTypes(m_type.m_dt, tv->m_type);
 }
 
 bool

hphp/test/quick/callable.php

@@ -5,9 +5,19 @@ function check($f) {
   var_dump(is_callable($f));
 }
 
+function typehint(callable $a) {
+  var_dump('worked');
+}
+
+function id() {}
+
 function main() {
     check('');
-    check('main');
+    check('id');
     check('blarblah');
+
+    $cl = function() { return 'closure'; };
+    typehint($cl);
+    typehint('id');
 }
 main();

hphp/test/quick/callable.php.expect

@@ -1,6 +1,8 @@
 string(0) ""
 bool(false)
-string(4) "main"
+string(2) "id"
 bool(true)
 string(8) "blarblah"
 bool(false)
+string(6) "worked"
+string(6) "worked"