Debugger:[Performance]Calculating 'typeof(Program)' in watch window takes more time

## Environment data
`dotnet --info` output:
VS Code version: 1.22.2
C# Extension version: 1.15.0-beta5

## Steps to reproduce

1. Download and open attachment in VSCode: [Thread.zip](https://github.com/OmniSharp/omnisharp-vscode/files/1941483/Thread.zip)
2. Set a breakpoint at line of 34 in Program.cs
3. F5
4. Add 'typeof(Program)' to watch window and expand this expression

## Expected behavior
The expression should be expanded immediately.

## Actual behavior
It takes about ten seconds to expand this expression.