Add the sqlite3_memdebug_title_count global variable, used during debugging

Add the sqlite3_memdebug_title_count global variable, used during debugging
to count the number of invocations of test_memdebug_settitle.  By examining
this variable in the debugger after a segfault, one can then set a breakpoint
on test_memdebug_settitle that will fire just before the problem.

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