Co-authored-by: Hack OSS Team <hhvm-oss@fb.com>

Co-authored-by: HHVM/Hack Open Source <hhvm-oss@fb.com>

- Variadic parameters were not checked
   function _(int ... $x): void {} now emits an error for $x.
- Lambda parameters are now linted in unparenthesized params.
   $x ==> 0 now emits an error for $x.
   Same applies for (int ... $x) ==> 0.
 - Added explicit tests for old style variadics.
   The linter handled them fine, but there was no test for it.
Checking for unparenthesized lambda parameters required a new linter.
The UnusedParameterLinter checked against ParameterDeclaration.
$x ==> 0; does not introduce one, so a new TNode was required.
This PR may cause old HHAST_FIXME comments to break.
You will have to upgrade:
/*HHAST_FIXME[UnusedParameter]*/ ($x) ==> 0; to
/*HHAST_FIXME[UnusedLambdaParameter]*/ ($x) ==> 0;
I suspect this linter is rarely suppressed, since the autofix is easier.

Co-authored-by: HHVM/Hack Open Source <hhvm-oss@fb.com>

Co-authored-by: HHVM/Hack Open Source <hhvm-oss@fb.com>