library/string.m:
Make each code unit in an ill-formed sequence contribute one
to the value of string.count_codepoints.
tests/hard_coded/Mmakefile:
tests/hard_coded/string_count_codepoints_ilseq.exp:
tests/hard_coded/string_count_codepoints_ilseq.exp2:
tests/hard_coded/string_count_codepoints_ilseq.m:
Add test case.
