Misleading Unused Variable Warnings if there's a syntax error in the file

**Bicep version**
Bicep CLI version 0.4.613 (d826ce8411)

**Describe the bug**
If there is a syntax error in the file making it unparseable, the unused variable warning can fire even if the variable is actually used. In the below example, the `accountName` variable most certainly is used, but I presume the usage isn't detected as a knock on from the parser error (lack of closing brace on the resource).

**To Reproduce**
Build the following bicep snippet
```bicep
var accountName = 'SomeName'
resource x 'Microsoft.Storage/storageAccounts@2021-04-01' = {
name: accountName
```

Start by reproducing the warning with the Bicep snippet in the issue and inspect the compiler diagnostics for the syntax error and unused-variable warning. Done means the incomplete resource still reports its syntax error without incorrectly warning that accountName is unused.

Written by the indexing model from the issue text.