Fix CA2101 triggering when BestFitMapping=false

The documentation for CA2101 indicates that the problem is not the use
of single-byte character marshalling, but that when doing so .NET will
by default try and substite characters that cannot be directly
marshalled.

Unfortunately, when a developer actually tries to use
BestFitMapping=false, such as when using ANSI-only APIs (which are
particularly common on non-Windows operating systems), CA2101 triggers
anyway and the developer has no choice but to suppress the Diagnostic or
disable the rule, adding noise to their code and reducing trust in the
analyzer.

CA2101 appears to work correctly in
Microsoft.CodeAnalysis.FxCopAnalyzers and in NetAnalyzers through
version 5.0.3. BestFitMapping=false only got disabled in version 6.0.0,
apparently due to a small mistake when refactoring to take the new
InvariantGlobalization setting into account.

This commit adds tests for BestFitMapping (which for some reason was
completely untested previously) and fixes the refactoring error.

src/NetAnalyzers/Core/Microsoft.NetCore.Analyzers/InteropServices/PInvokeDiagnosticAnalyzer.cs

@@ -121,7 +121,7 @@ public void AnalyzeSymbol(SymbolAnalysisContext context)
                 }
 
                 // CA2101 - Specify marshalling for PInvoke string arguments
-                if (dllImportData.BestFitMapping != false ||
+                if (dllImportData.BestFitMapping != false &&
                     context.Options.GetMSBuildPropertyValue(MSBuildPropertyOptionNames.InvariantGlobalization, context.Compilation) is not "true")
                 {
                     bool appliedCA2101ToMethod = false;

src/NetAnalyzers/UnitTests/Microsoft.NetCore.Analyzers/InteropServices/PInvokeDiagnosticAnalyzerTests.cs

@@ -640,6 +640,43 @@ End Class
                 BasicResult2101(22, 30));
         }
 
+        [Fact]
+        public async Task CA2101WithoutBestFitMappingCSharpTestAsync()
+        {
+            await VerifyCS.VerifyAnalyzerAsync(@"
+using System.Runtime.InteropServices;
+using System.Text;
+
+class C
+{
+
+    [DllImport(""user32.dll"", BestFitMapping = false)]
+    private static extern void Method1(string s);
+
+    [DllImport(""user32.dll"", CharSet = CharSet.Ansi, BestFitMapping = false)]
+    private static extern void Method2(string s);
+
+    [DllImport(""user32.dll"", BestFitMapping = false)]
+    private static extern void Method3(StringBuilder s);
+
+    [DllImport(""user32.dll"", CharSet = CharSet.Ansi, BestFitMapping = false)]
+    private static extern void Method4(StringBuilder s);
+
+    [DllImport(""user32.dll"", BestFitMapping = false)]
+    private static extern string Method5();
+
+    [DllImport(""user32.dll"", BestFitMapping = false)]
+    private static extern StringBuilder Method6();
+
+    [DllImport(""user32.dll"", CharSet = CharSet.Ansi, BestFitMapping = false)]
+    private static extern string Method7();
+
+    [DllImport(""user32.dll"", CharSet = CharSet.Ansi, BestFitMapping = false)]
+    private static extern StringBuilder Method8();
+}
+");
+        }
+
         #endregion
     }
 }