Handle explicitly nullable types in optional-before-required deprecation

The exception for null default values here exists to keep compatibility
with PHP < 7.1 where "Foo $bar = null" wa

Handle explicitly nullable types in optional-before-required deprecation

The exception for null default values here exists to keep compatibility
with PHP < 7.1 where "Foo $bar = null" was the canonical way to create
a nullable parameter. If the parameter is actually "?Foo $bar = null",
then clearly compatibility with PHP < 7.1 is not a concern, and we
can throw a deprecation notice.

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Consistently treat optional-before-required as required

There was a loophole here when it came to usage with named arguments,
which was not intended. Close the loophole thoroughly by act

Consistently treat optional-before-required as required

There was a loophole here when it came to usage with named arguments,
which was not intended. Close the loophole thoroughly by actually
dropping the default value from the signature entirely. The default
is still used to make the type nullable, but not for anything else.

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Deprecate required param after optional

As an exception, we allow "Type $foo = null" to occur before a
required parameter, because this pattern was used as a replacement
for nullable

Deprecate required param after optional

As an exception, we allow "Type $foo = null" to occur before a
required parameter, because this pattern was used as a replacement
for nullable types in PHP versions older than 7.1.

Closes GH-5067.

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