Improved C++ name-demangling code

Moved the C++ name-demangling code from commit 5bd61a55a0e24a360c410f3282ab593aa8fc76c0 into a separate function that's called only at output time.  This change has two advantages: (1) It should improve performance because demangling is performed once per function, not once per function invocation, and (2) it doesn't interfere with the BYFL_CALLEE `Byfl` column, which was always returning `no` because of mismatched string comparisons caused by demangling.