Rename SemaBounds EqualExpr* types and methods to ExprSet*

This issue was copied from https://github.com/microsoft/checkedc-clang/issues/972

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The free variables work still uses an `EqualExprTy` to denote the type of a set of expressions. There is also the method `EqualExprSetContainsExpr` that operates on an `ExprSetTy` and so should be renamed to `ExprSetContainsExpr`.