Port MoodlePHPdoc check function types check

[james-cnz]

This is a PHPDoc types check intended to be similar in scope to the MoodlePHPdoc check one.

It is a bit more permissive in a few respects:

• It makes some basic effort to allow PHPDoc types to be subtypes of the native type. (I believe this is standard to PHPDoc types.)
• It allows mixed type arrays, e.g. (a|b)[] . (This is supported by PHP-FIG PSR-5.)
• It allows PHPDoc param annotations to contain pass by reference or splat.

But, AFAIK, it is only stricter in one way:

• If the native type contains null, the PHPDoc type must too. [EDIT: Sorry, I mean the other way round. If the Doc type contains null, the native type must too.]
  [EDIT: It looks like the existing checks fail to check PHPDocs that are separated from the function by an attribute, so it's also stricter in that way.]

As with the existing test, it makes no effort to syntax check types, check namespaces, match return types, or do any checking on variables.
And it passes existing tests.
So I don't think it should cause too much disruption.

[james-cnz]

I forgot about parentheses in types.

[james-cnz]

I've made the type simplification aware of well-formedness of types. It doesn't make much difference now, but would make it easier to include a check that types are well-formed in the future, if desired.

[james-cnz]

I forgot about the type "$this".

[james-cnz]

I've improved subtype recognition a bit, to support, e.g. MDL-83375.

[james-cnz]

Improved error messages a bit, to help address MDLSITE-3694.