Subclass of range_label for labelling the type of EXPR when reporting
a type mismatch between EXPR and OTHER_EXPR.
Either or both of EXPR and OTHER_EXPR could be NULL.
Implementation of range_label::get_text for
maybe_range_label_for_tree_type_mismatch.
If both expressions are non-NULL, then generate text describing
the first expression's type (using the other expression's type
for comparison, analogous to %H and %I in the C++ frontend, but
on expressions rather than types).
References EXPR_P, range_label_for_type_mismatch::get_text(), ggc_alloc(), m_expr, m_other_expr, NULL, NULL_TREE, and TREE_TYPE.