IL parser should detect non-constant strings (ldstr) passed to Get*String calls

[abock]

The IL parser can extract false-positives if the Catalog.Get*String APIs are misused. To avoid this, and to warn of improper usage of the API, the IL parser should warn if non-constants are passed to the APIs.

Example:

void Foo()
{
    var x = "Foo";
    DoSomething("Hello");
    Catalog.GetString(x);
}

Vernacular may actually extract the constant ldstr "Hello" instruction that is an argument for DoSomething because x (a non-constant load) is passed to GetString.

Fixing this addresses two issues:

• no false-positive string extractions due to possible "bleeding" of ldstr instructions
• catch improper usage of the API where non-constants are being passed (and thus not possible to extract)

[StephaneDelcroix]

I totally second this, but it would require an additional API call to do Catalog.GetString(x) on purpose, used only at runtime and not parsed