FAILURE: Could not locate Microsoft.CodeAnalysis.VisualBasic.Workspaces.dll

## Issue Description ##
We have an extremely large solution circa 130 projects which I assume is pushing the performance bounderies of Omnisharp. Intellisense is flakey and often completely unresponsive.

The error in the title is one of several 'could not locate' errors.

## Steps to Reproduce ##
Open vs code, wait for Omnisharp to finish loading

## Expected Behavior ##
Intellisense works and is responsive. Problems are highlighted in near realtime after initial loading period.

## Actual Behavior ##
Intellisense is flakey or totally unresponsive. Problems are only highlighted after a build.

## Logs ##
N/A

### OmniSharp log ###
[omnisharp.log](https://github.com/OmniSharp/omnisharp-vscode/files/4761387/omnisharp.log)

### C# log ###
N/A

## Environment information ##

**VSCode version**: 1.46.0
**C# Extension**: 1.22.0

Mono Information
OmniSharp using built-in mono
Dotnet Information
.NET Core SDK (reflecting any global.json):
Version: 3.1.202
Commit: 6ea70c8dca

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.15
OS Platform: Darwin
RID: osx.10.15-x64
Base Path: /usr/local/share/dotnet/sdk/3.1.202/

Host (useful for support):
Version: 3.1.4
Commit: 0c2e69caa6

.NET Core SDKs installed:
3.1.202 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.7 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.7 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.4 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.7 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|cmake-test-adapter|derivitec-ltd|0.7.2|
|cmake-tools|ms-vscode|1.4.0|
|cpptools|ms-vscode|0.28.3|
|csharp|ms-dotnettools|1.22.0|
|debug|webfreak|0.25.0|
|screen-recorder|wk-j|0.0.2|
|vetur|octref|0.24.0|
|vscode-dotnet-adapter|derivitec-ltd|1.3.3|
|vscode-lldb|vadimcn|1.5.3|
|vscode-solution-explorer|fernandoescolar|0.3.10|
|vscode-test-explorer|hbenl|2.19.1|;