Treat generated column number as 0-based in indexed originalPositionFor

[motiz88]

Currently originalPositionFor behaves differently in BasicSourceMapConsumer and IndexedSourceMapConsumer:

• In the former, the column argument is (correctly and as documented) treated as 0-based.
• In the latter, the column argument is treated as 1-based for the purpose of locating the section in which the generated position occurs (as the internal section.generatedOffset.generatedColumn field is, for whatever reason, 1-based).

This is a longstanding issue, going back to the introduction of IndexedSourceMapConsumer AFAICT, but the behavior is incorrect. This PR fixes the issue and adds a corresponding test.

[coveralls]

Pull Request Test Coverage Report for Build 547

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.385%

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Totals
Change from base Build 546:	0.0%
Covered Lines:	824
Relevant Lines:	911

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💛 - Coveralls