unary not is fooling tracker of variable use when assignment is in foreach loop

Steps to reproduce

Get-AzAutomationSchedule $resourceGroup $automationAccount | ForEach-Object {
            $PrimaryExists = $true
}

if (-not $PrimaryExists) {
    Write-Output "hello"
}

Expected behavior

$PrimaryExists should not be flagged as assigned but never used.

Actual behavior

It is flagged.

Environment data

> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      6.2.3
PSEdition                      Core
GitCommitId                    6.2.3
OS                             Linux 4.15.0-1056-oem #65-Ubuntu SMP Thu Sep 12…
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

> (Get-Module -ListAvailable PSScriptAnalyzer).Version | ForEach-Object { $_.ToString() }
1.18.3

Notes

I tried to create a more concise example, but the assignment needs to be a foreach { } at the end of a pipeline to exhibit the error.

Start by running the supplied PowerShell pipeline and checking the analyzer diagnostic for the assignment inside the trailing ForEach-Object block. Trace the variable-use analysis that produces the assigned-but-unused warning, then add a regression test for this example and confirm the warning is no longer reported while ordinary unused assignments remain flagged.