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.