fix isequal_normalized for combining-char reordering (#52447)
Fixes #52408.
(Note that this function was added in Julia 1.8, in #42493.)
In the future it would be good to further optimize this function by
adding a fast path for the common case of strings that are mostly ASCII
characters. Perhaps simply skip ahead to the first byte that doesn't
match before we begin doing decomposition etcetera.
