Improve ternary type inference

[jeskew]

Resolves #15397

Worked on this as an alternative way to address #15114 (instead of #15398). This PR does two things:

1. If a ternary has a literally-typed condition (i.e., it is definitely true or definitely false), the type engine will use the type of the active branch's expression. E.g., the type of true ? 'a' : 'b' is 'a', and the type of false ? 'a' : 'b' is 'b'. There was a TODO comment in TypeAssignmentVisitor suggesting this change.
2. If the types of both branches can be combined instead of represented as a union, the type engine will do so. For example, the type of unknownCondition ? 'a' : 'b' is 'a' | 'b' (a union), but the type of unknownCondition ? [stringParam] : [intParam] is [int | string] (assuming the type of stringParam is string and the type of intParam is int). This change relies on existing type collapsing logic, so it will handle things like combining refinements on string types and combining objects into tagged unions if possible.

One change I made to the TypeCollapser is to collapse objects that can't be combined into a tagged union into an object whose properties are a union of the possible property types of the inputs. This is similar to how we collapse tuple types. Given a template like the following:

param a {
  foo: string
}

param b {
  bar: string
  *: int
}

param condition bool

var value = condition ? a : b

value would have a type of { bar: string, foo: int | string, *: int }, with all properties flagged as optional.

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test this change out locally with the following install scripts (Action run 11536289165)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 11536289165

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 11536289165"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 11536289165

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 11536289165"

[github-actions]

Dotnet Test Results

    72 files   -     36      72 suites   - 36   29m 18s ⏱️ - 14m 16s
11 376 tests  -     22  11 376 ✅  -     22  0 💤 ±0  0 ❌ ±0 
26 483 runs   - 13 228  26 483 ✅  - 13 228  0 💤 ±0  0 ❌ ±0 

Results for commit 252c1f7. ± Comparison against base commit 48315b8.

[shenglol]