fix(combineLatest): improve typings for combineLatest

[benlesh]

Existing typings did a poor job of getting proper types from code that passed an array of observables or observable inputs to combineLatest. This issue was caught while trying master in google3. This resolves that issue.

[kwonoj]

looks ok, but would love to get second eye.

[coveralls]

Pull Request Test Coverage Report for Build 7897

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 96.966%

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Files with Coverage Reduction	New Missed Lines	%
src/internal/observable/fromPromise.ts	1	87.5%
src/internal/observable/fromIterable.ts	2	95.24%
src/internal/observable/fromObservable.ts	2	83.33%

Totals
Change from base Build 7887:	0.2%
Covered Lines:	5784
Relevant Lines:	5965

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💛 - Coveralls