fix(compiler): work around TypeScript bug when narrowing switch statements

[crisbeto]

We type check @switch blocks by generating identical TS switch statements in the TCB, however TS currently has a bug where parenthesized switch block expressions don't narrow their types. Since we use parenthesized expressions to wrap AST nodes for diagnostics, this will bug will affect all Angular-generated switch statements.

These changes work around the issue by generating if/else if/else statements that represent the switch.

Some alternatives that were considered:

1. Moving the switch expression to a constant - this is fairly simple to implement, but it won't fully resolve the narrowing issue since the same constant will have to be used in expressions inside the different cases.
2. Removing the outer-most parenthesis from the switch expression - this works and allows us to continue using switch statements, but because we use parenthesized expressions to map diagnostics to their template locations, I wasn't sure if it won't lead to worse template dignostics.

Fixes #52077.

[cexbrayat]

@crisbeto I wanted to bring to your attention #52107 where I was asking about the possibility of having exhaustive switches in templates. If the TCB generates if statements, will that be possible to achieve?

[JoostK]

@crisbeto I wanted to bring to your attention #52107 where I was asking about the possibility of having exhaustive switches in templates. If the TCB generates if statements, will that be possible to achieve?

Yep, no problem:

if (x.type === 'foo') {}
else if (x.type === 'bar') {}
else {
  const _: never = x.type;
}

will report an error if x.type has not been narrowed to the never type. I'm not sure whether the reported error message would be clear enough though.

[cexbrayat]

@JoostK Ha yes, the never trick 👍 I hope this lands one way or another, I think it would be a great feature. Thanks for the answer!

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[crisbeto]

Reworked based on Joost's feedback.

[atscott]

This PR was merged into the repository by commit 386e1e9.

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