Support Match nodes with non-int-literals in the back-end.

[sjrd]

See scala/scala#8451 upstream (not yet merged, but fully approved)

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Since Scala 2.13.2, the pattern matcher will keep Match nodes that match on Strings and nulls as is, to be desugared later in cleanup. This was implemented upstream in scala/scala#8451

We implement a more general translation that will accept any kind of scalac Literal. If the scrutinee is an integer and all cases are int literals, we emit a js.Match as before. Otherwise, we emit an if..else chain with === comparisons.

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Locally tested with:

> set resolvers in Global += "scala-pr-validation" at "https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots/"
> ++2.13.2-bin-9f25ca0-SNAPSHOT
> testSuite/test
> compiler/test
[...] copy the big test case from the PR in HelloWorld.scala and:
> helloworld/run

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As noted at scala/scala#8451 (comment), 0.6.x already works out of the box with the change upstream.

It actually raises the question whether we should revert the design of our js.Match to accept any kind of js.Literals and really iron out the linker (notably for IR literals that are not JS literals, such as ClassOf and LongLiteral) instead of changing the compiler back-end. This PR implements the less disruptive change.

[gzm0]

It actually raises the question whether we should revert the design of our js.Match to accept any kind of js.Literals and really iron out the linker (notably for IR literals that are not JS literals, such as ClassOf and LongLiteral) instead of changing the compiler back-end.

We should keep this as a a consideration, especially as it would simplify the emitted code IIUC.