C#: Special handling of unknown types in isMatchingConstant.

[michaelnebel]

In this PR we remove some false positives for various queries. The completion logic lead to wrong conclusions on matching (when unknown types are involved, the completion logic derived that a pattern would never match), which also affects the control flow graph succ/pred logic.
It appears that it is mostly the cs/constant-condition and cs/useless-assignment-to-local for BMN databases that are affected by this (this problem became more evident after #18894 as we categorise more types as unknown).

The DCA results for the powershell project were spot-checked and the removed results indeed appeared to be false positives. Prior to the change the matching against an unknown type would lead to cs/constant-condition and typically a subsequent cs/useless-assignment-to-local for cases like

if (x is MyUnknownType y) {
   Use(y);
}

[Copilot]

PR Overview

This PR refines the matching logic for unknown types in the isMatchingConstant function to eliminate false positives, particularly those related to constant conditions and useless assignments.

• Adjusts handling of expressions that involve unknown types.
• Updates a test case to assert that unknown types do not trigger a constant condition alert.

Reviewed Changes

File	Description
csharp/ql/test/query-tests/standalone/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.cs	Adds test cases to verify that a known type (int) triggers an alert while an unknown type (D) does not

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

csharp/ql/test/query-tests/standalone/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.cs:21

• The type 'D' is used without a definition, which may cause compilation issues. If this is intentional to simulate an unknown type, consider adding a comment clarifying this or provide a dummy definition of D.

var d = new D();

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[hvitved]

Nice!

[tamasvajk]

How do we reference buildess DBs in the docs, should it be build-mode: none?

[michaelnebel]

I will change it to that.
Maybe the abbreviation BMN is also acceptable? (What do you think @coadaflorin? )