[clang][dataflow] Diagnose branch conditions using the data-flow framework

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I'm currently working on a prototype dead-code check using the data-flow framework: #139068
The check works by finding loop conditions that are always true or always false, and emitting an error there.
However, the dataflow framework's Diagnoser entity doesn't have the ability to diagnose specifically loop conditions. It only has the ability to diagnose expressions.
(The workaround for this is to tag the loop conditions separately, but it's most definitely not a sound workaround, since the tags are also visible in unexpected places.)

There needs to be a way to diagnose specifically loop conditions, in a way that is known to the Diagnoser.
Ideally this diagnosis would be done in a way where both the normal Expr diagnoser, and the condition diagnoser are part of the same function.

Currently I'm thinking to add another overload to operator() in the Diagnoser class, into the DiagnosisCallback here:

llvm-project/clang/include/clang/Analysis/FlowSensitive/DataflowAnalysis.h

Lines 199 to 202 in 904d0c2

	template <typename AnalysisT, typename Diagnostic>
	using DiagnosisCallback = llvm::function_ref<llvm::SmallVector<Diagnostic>(
	const CFGElement &, ASTContext &,
	const TransferStateForDiagnostics<typename AnalysisT::Lattice> &)>;

But at that point it might be better to move the branch-diagnoser into a named function inside the same class.

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What do you think @martinboehme @Xazax-hun @jvoung ?