PR20416: handle @entry(@perf("..."))

PR20416: handle @entry(@perf("..."))

The dwarf_var_expanding_visitor::visit_entry_op function inherited
some old logic from expanding $var's in .function().return probes.
This logic is not approproate for the more formal way by which current
code synthesizes a .call probe to collect @entry(EXPR) values.  Now we
pass the EXPR tree to the .call probe unmodified, for resolution
there, rather than in a fake non-.return context here.  This makes
@entry(@perf("...")) expressions work.