Enforced argument and return types in QueryBuilder and ExpressionBuilder

[morozov]

Q	A
Type	improvement
BC Break	yes

1. Some of the ExpressionBuilder method arguments were mistakenly marked as mixed and weren't automatically converted to type declarations. A similar type enforcement was done in Modified AbstractPlatform::getLocateExpression() and ::getSubstringExpression() signatures #3494 and Reworked AbstractPlatform::get*Expression() methods #3498.
2. The QueryBuilder methods returning the instance are enforced to return self.

[greg0ire]

Looks good but I'm starting to think that maybe some docblocks could be removed entirely… not sure how much value is brought by comments such as "The left expression.". We could use $left and $right instead of $x and $y

[morozov]

Frankly speaking, I'm not even sure why this comparison() method is needed at all. It just concatenates all given values and adds a space in between. In the rest of the cases, the names of the arguments would be more obvious.