String array item does not allow browsing documention for override methods.

## Issue Description ##
An item of a string array and a string do not show the same IntelliSense. In case of string array, I cannot browse through overrides. In the example below, `test[0]` is a string, but typing `TrimStart(` does not show the documentation, unlike `test2` which is a string. I had to press Ctrl + I, Ctrl + K to see the documentation, and even then, I could not see the documentation for each of the 3 override methods.


## Steps to Reproduce ##
Type the following code and try to see the documentation for `TrimStart`.
```
string[] test = new string[]{"hello", "world"};
string test2 = "hello";
//test2.TrimStart()
test[0].TrimStart()
```

## Expected Behavior ##
Both `test2` and `test[0]` should behave the same way.

## Actual Behavior ##
`test[0]` behaves differently.

## Logs ##

### OmniSharp log ###
Post the output from Output-->OmniSharp log here

### C# log ###
Post the output from Output-->C# here

## Environment information ##

**VSCode version**: 1.50.1
**C# Extension**: 1.23.4

Mono Information
OmniSharp using built-in mono
Dotnet Information
.NET Core SDK (reflecting any global.json):
Version: 3.1.403
Commit: 9e895200cd

Runtime Environment:
OS Name: ubuntu
OS Version: 20.04
OS Platform: Linux
RID: ubuntu.20.04-x64
Base Path: /usr/share/dotnet/sdk/3.1.403/

Host (useful for support):
Version: 3.1.9
Commit: 774fc3d6a9

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

.NET Core runtimes installed:
Microsoft.AspNetCore.App 3.1.9 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.9 [/usr/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|
|---|---|---|
|cpptools|ms-vscode|1.0.1|
|csharp|ms-dotnettools|1.23.4|;