Improve TruncateTime canonical function performance

[brandondahler]

Ported from CodePlex, taking back over from #467 .

Updates TruncateTime function from

TruncateTime(DateTime X)	        
    PreKatmai:    TRUNCATETIME(X) => CONVERT(DATETIME, CONVERT(VARCHAR(255), expression, 102),  102)
    Katmai:    TRUNCATETIME(X) => CONVERT(DATETIME2, CONVERT(VARCHAR(255), expression, 102),  102)
TruncateTime(DateTimeOffset X)
    TRUNCATETIME(X) => CONVERT(datetimeoffset, CONVERT(VARCHAR(255), expression,  102) + ' 00:00:00 ' +  Right(convert(varchar(255), @arg, 121), 6),  102)

to

TruncateTime(DateTime X)
PreKatmai:    TRUNCATETIME(X) => DATEADD(d, DATEDIFF(d, 0, expression), 0)
Katmai:       TRUNCATETIME(X) => CAST(CAST(expression AS DATE) as DATETIME2)

TruncateTime(DateTimeOffset X)
Katmai only: TRUNCATETIME(X) => TODATETIMEOFFSET(CAST(expression AS DATE), DATEPART(tz, expression))

[brandondahler]

Addressed latest nits, fixed broken tests, and rebased upon current master. Recommend closing #467 in favor of this PR.

[brandondahler]

Honestly not sure why the test returns one way on my machine and another on the CI machine. Reverting the test change. I don't believe that SqlServer CE has CAST(... as Date)

[ajcvickers]

Thanks @brandondahler for the contribution!

[vslee]

resolves #824