C++: Fix FP in cpp/overflow-buffer

[MathiasVP]

We "handled" reference types in one disjunct of getSize, but not in the other (where by "handle" I mean "excluded"). This caused some FPs when capturing the destination variable by reference in a lambda.
This PR fixes that by properly excluding reference types as sources in both disjuncts.

Commit-by-commit review recommended.

DCA looks uneventful.

[Copilot]

Pull Request Overview

This PR fixes a false positive in the cpp/overflow-buffer query by properly handling reference types. The issue was that reference types were excluded from size calculations in one part of the getSize predicate but not in another, causing false positives when capturing destination variables by reference in lambdas.

• Extracts reference type handling into a shared helper predicate getVariableSize
• Updates both disjuncts of getSize to consistently exclude reference types from size calculations
• Adds a test case demonstrating the fixed scenario with lambda capture by reference

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Buffer.qll	Adds getVariableSize helper and updates getSize to consistently exclude reference types
tests.cpp	Adds test case test31 demonstrating lambda capture scenario that should not trigger false positive
UnboundedWrite.expected	Updates expected test results with corrected line numbers after adding new test case

[jketema]

LGTM