Backport to the 3.7.15 branch 
the fix to the segfault problem of ticket [a7b7803e8d1e869] which involved
the use of "AS" named result columns as logical terms of the WHERE clause.
Also, change the version number to 3.7.15.1.
Version 3.7.15.1
Fix an over-aggressive optimization of ORDER BY on a three-way join where the
second table is UNIQUE and the ORDER BY occurs on the third table.
Ticket [598f5f7596b0557].
Version 3.7.15.2 release candidate
Add a single test case to fts4unicode.test to verify that title-case
maps to lower case.