Syntax error markers do not go away after the issue is corrected

System Details

OS: Windows 10 Enterprise 
Version         InstallDate                     OSArchitecture
-------          -----------                    --------------
10.0.14393         1/26/2017 7:52:53 PM                    64-bit

code -v:
1.13.1
379d2efb5539b09112c793d3d9a413017d736f89

$pseditor.EditorServicesVersion:
Major  Minor  Build  Revision
-----  -----  -----  --------
1      4      0      0

Extensions: 
DotJoshJohnson.xml@1.9.1
jchannon.csharpextensions@1.3.0
jmrog.vscode-nuget-package-manager@1.1.3
MS-DevSkim.vscode-devskim@0.1.4
ms-vscode.csharp@1.10.0
ms-vscode.PowerShell@1.4.0
ms-vsts.team@1.119.0
qinjia.seti-icons@0.1.3
Tyriar.theme-sapphire@0.2.1

$PSVersionTable:
Name                           Value
----                           -----
PSVersion                      5.1.14393.1358
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.14393.1358
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Issue Description

VSCode error parser got stuck showing an error on a piece of code that had been corrected. Would not clear until after VSCode was closed/reopened.

Start by reproducing the reported VSCode error-parser behavior: show a syntax error, correct the code, and observe whether the marker remains until restart. Trace the parser and diagnostic update flow in the PowerShell extension; done means markers clear promptly after the underlying issue is corrected, without closing and reopening VSCode.