Support LIKE with nondeterministic collations

This patch adds support for using LIKE with nondeterministic collations.
So you can do things such as

col LIKE 'foo%' COLLATE case_insensitive

This currently results in a "not supported" error. The reason for that
is that when I first developed support for nondeterministic collations,
I didn't know what the semantics of that should be, especially since
with nondeterministic collations, strings of different lengths could be
equal, and then dropped the issue for a while.

After further research, the SQL standard's definition of the LIKE
predicate actually provides a clear definition of the semantics: The
pattern is partitioned into substrings at wildcard characters (so
'foo%bar' is partitioned into 'foo', '%', 'bar') and then then whole
predicate matches if a match can be found for each partition under the
applicable collation (so for 'foo%bar' we look to partition the input
string into s1 || s2 || s3 such that s1 = 'foo', s2 is anything, and s3
= 'bar'.) The only difference to deterministic collations is that for
deterministic collations we can optimize this by matching by character,
but for nondeterministic collations we have to go by substring.