[windows] Enable tests for LLDB in swift-main-windows-toolchain

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The infrastructure for running a stable subset of LLDB tests on Windows is in place on mainline. We enabled it for PR-testing with #78844, but https://ci-external.swift.org/job/swift-main-windows-toolchain/ uses a custom (hardcoded) invocation that I cannot access.

Bringing LLDB testing to swift-main-windows-toolchain seems important to monitor flakes. We filtered loads of flaky test in the bringup phase, but future changes may introduce new ones and it's critical to find them early. Otherwise they might clog the PR-testing bots.

Right now, it invokes swift\utils\build.ps1 directly and passes -Test swift,dispatch,xctest. We could either extend it to -Test swift,dispatch,xctest,lldb or drop the parameter entirely so that build.ps1 selects the default.

[weliveindetail]

@shahmishal What do you think?

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While we are here, we should also add -Summary to monitor build and tests times.

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New test flake that popped up randomly in one of my own PRs: #79154 We should get better monitoring here.

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@shahmishal Gentle ping. Is there anything I can do to help with this task?