Normative: explicitly specify order of operations in MakeTime

[bakkot]

While reviewing #2007, @michaelficarra and I noticed that MakeTime is underspecified: it has a step which reads

Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according to IEEE 754-2019 rules (that is, as if using the ECMAScript operators * and +)

but does not specify in which order to perform the additions. The choice of order is observable when using IEEE double-precision floats: for example,

• ((80063993375 * 3600000 + 29 * 60000) + 1 * 1000) + -288230376151711744 is 29312, but
• 80063993375 * 3600000 + (29 * 60000 + (1 * 1000 + -288230376151711744)) is 29248.

(The actual value is 29256.) So Date.UTC(1970, 0, 1, 80063993375, 29, 1, -288230376151711740) can have either of those results depending on your choice of order.

SpiderMonkey, ChakraCore, V8, and XS all appear to perform the arithmetic in left-to-right order, producing 29312. Graal produces 29248. (JSC bounds all arguments above by 2^31, so it cannot observe this case, and quickJS produces 29256 because it internally uses 64-bit integers for this arithmetic, rather than the IEEE floats required, and so does not lose precision.)

In this PR I've chosen to go with the majority and enforce left-to-right evaluation order.

[jmdyck]

The phrase "as if using the ECMAScript operators * and +" suggests to also use ECMAScript associativity, which agrees with this PR.

[bakkot]

Tests: tc39/test262#2831