Avoid unnecessary out-of-scope file promotion when the cursor results in no declaration

[PathogenDavid]

Out-of-scope file promotion happens eagerly in TranslationUnitParser.CreateDeclarations:

https://github.com/InfectedLibraries/Biohazrd/blob/99a504b77c076651ade931aa168ce7359adc0a4a/Biohazrd/TranslationUnitParser.cs#L251-L253

An unfortunate side-effect of this is if the cursor results in no translated declarations, the file is still promoted and the warning is still emitted.

This was discovered with this workaround (see #171 for some background.)

[PathogenDavid]

Repro test for ScopeTests:

        [Fact]
        [RelatedIssue("https://github.com/InfectedLibraries/Biohazrd/issues/172")]
        public void OutOfScopeIgnoredDeclarationInsideInScopeDeclarationIsNotInScope()
        {
            TranslatedLibraryBuilder builder = new();
            builder.AddFile(new SourceFile("A.h")
            {
                Contents = "struct __declspec(dllexport) MyStruct;",
                IsInScope = false,
                IndexDirectly = false
            });

            builder.AddFile(new SourceFile("B.h")
            {
                Contents = @"
#include ""A.h""

struct MyStruct
{
    int x;
};
"
            });

            TranslatedLibrary library = builder.Create();

            Assert.Empty(library.ParsingDiagnostics);
            Assert.Single(library.Files);
            Assert.Single(library.Declarations);
            Assert.Equal("MyStruct", library.Declarations[0].Name);
        }