Refactored to make it clear when `@operator.Symbol as IMethodSymbol` can be `null`.

…ion are declared before they are used.

Factor `ClauseCall` out into three classes to make it clear when the fields
`operand` and `declaration` can be `null`.

Using the `forceCachingInSameStage()` trick, we can get rid of the non-cached version
of local flow, while still computing it in the same stage.

…brary

Computing strings and locations for CIL instructions can be quite time consuming.
The CIL `toString()`s are not very helpful in path explanations, and their locations
are only useful when a PDB source file exists. Therefore, produce a simple constant
`toString()`, and restrict locations to those in PDB files.

As described on https://lgtm.com/help/ql/locations#providing-location-information,
there is no need to provide a `getUrl()` predicate, when there is a `getLocation()`
predicate. Not only is it redundant, but it can also be slow because of string
construction.

…orings

C#: nullness related extractor refactorings

C#: Cache `NamedElement::getLabel()`

Java: Add missing override annotations.

Java: Remove `Metrics/queries.xml`

CPP: Improve qldoc for MacroAccess and MacroInvocation.