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Deduplicates the setting up of the `zend_string_init_interned()` call, removes
the need for `ExposedDocComment::getLength()` and so that method is removed.

* Return a string rather than an array, all callers just immediately used
`implode()` to join the elements in the array with nothing between them
* In the callers, inline some single-use variables with the template for the
version-dependent code
* Remove the callback to `array_filter` specifying that only items that are not
`empty()` be removed - this is the default behavior

There is no need to add special handling for the default value of `null`, since
it is not loosely-equals to any of the strings 'UNKNOWN', 'false', 'true', or
'null' it will just be returned directly anyway.

Move the logic out of `funcInfoToCode()` and update it. In the process, make
`ArgInfo::getDefaultValueAsArginfoString()` private.

The vast majority of the decisions about the use of `ZEND_BEGIN_ARG_INFO_EX` or
one of its variations are based on the return information of the function - is
the type builtin, is the return information tentative, does it include an
object mask, etc. Accordingly, move the logic into the `ReturnInfo` class.

The logic is actually moved into two methods, `ReturnInfo::beginArgInfo()`,
which needs to handle the case of tentative returns being used when PHP < 8.1
is supported, and `::beginArgInfoCompatible()`, which can assume that PHP 8.1+
is supported and thus make use of early returns and guard clauses. Further
improvements to the logic will be made in a subsequent commit.

In the process, make `ReturnInfo::$byRef` private.

Reduce the number of global functions by moving it to instance method
`FuncInfo::toArgInfoCode()`.

In the process, make `FuncInfo::$numRequiredArgs` private.

The following parameters were either unused before this commit or became unused
as part of updating callers to stop passing unused parameters to other
functions updated in this commit:
* `FuncInfo::getMethodSynopsisDocument()` - `$funcMap`, `$aliasMap`
* `FuncInfo::getMethodSynopsisElement()` - `$funcMap`, `$aliasMap`
* `ConstInfo::getGlobalConstDeclaration()` - `$allConstInfos`
* `generateMethodSynopses()` - `$aliasMap`
* `replaceMethodSynopses()` - `$aliasMap`

* Use `@param` instead of `@var` for parameters
* Fix type of `$attributeGroups` in `AttributeInfo::createFromGroups()`
* Remove extra documentation of `$allConstInfo` for
`ClassInfo::getClassSynopsisDocument()`, it is already documented under the
correct name `$allConstInfos`
* Remove unneeded `@throws`

Reduce the number of global functions by moving it to instance method
`FileInfo::handleStatements()`.

Reduce the number of global functions by moving it to static method
`FileInfo::handlePreprocessorConditions()`. Since it is only used by
`FileInfo::handleStatements()`, also make it private.

Reduce the number of global functions by moving it to static method
`DocCommentTag::parseDocComments()`.

For a lot of the structures, the parsing of doc comment tags is based on if a
specific tag is present, or the value that it has if it is. Add a new helper
method, `DocCommentTag::makeTagMap()`, that turns an array of tag instances
into a map from tag name to value (the last value, if there are multiple uses
of the same tag name). Then, for the simple cases where just a tag's presence
is all that is checked, or just the (last) value is used, check the map instead
of using a loop through all of the tags present.

Separate out the creation of a legacy version of a FileInfo object, which has
information for old versions of PHP discarded, from its subsequent use in
`processStubFile()`.

In the process, make `FileInfo::$legacyArginfoGeneration` private, and inline
the single use of `FileInfo::getAllClassInfos()`, removing that method.

The following properties are made private:
* `ArgInfo::$phpDocType`
* `ClassInfo::$flags`, `::$attributes`, `::$extends`, `::$implements`
* `FileInfo::$isUndocumentable`

The following are made protected:
* `VariableLike::$flags`

Reduce the number of global functions by moving it to static method
`FileInfo::parseStubFile()`. Additionally, make `FileInfo::handleStatements()`
private now that the only caller is part of the class.