`IAssemblySymbol.MightContainExtensionMethods` produces unstable result for a source assembly

`IAssemblySymbol.MightContainExtensionMethods` produces unstable result for a source assembly.
Initially, it will return `true`. But once the final determination is made, it returns the actual result (whether the assembly did in fact contain extensions).
We should consider forcing the processing to complete when this public API is called.

```
[Fact]
public void MightContainExtensionMethods_01()
{
var comp = CreateCompilation("");
Assert.True(comp.SourceAssembly.GetPublicSymbol().MightContainExtensionMethods);
comp.VerifyEmitDiagnostics();
Assert.False(comp.SourceAssembly.GetPublicSymbol().MightContainExtensionMethods);
}
```

Start at the public IAssemblySymbol.MightContainExtensionMethods API and trace how a source assembly reaches its final extension-method determination. Run or adapt the shown MightContainExtensionMethods_01 test, and consider the work complete when the API's result is deterministic and the test verifies the intended behavior before and after compilation processing.

Written by the indexing model from the issue text.