Add attribute to support inline namespaces in C++/CLI. (#781)

* Add attribute to support inline namespaces in C++/CLI.

A new assembly-scope custom attribute

[CppInlineNamespace(string dottedName)]

has been added to implement inline namespaces in C++/CLI. For every
inline namespace encountered in a translation unit, the C++ compiler
will emit this attribute with the fully scoped name, in CLR dotted
form, as the argument for the attribute.

* Add [CppInlineNamespace] to refs.

Author: tgani-msft

src/libraries/System.Runtime.CompilerServices.VisualC/ref/System.Runtime.CompilerServices.VisualC.cs

@@ -66,6 +66,11 @@ public sealed partial class NativeCppClassAttribute : System.Attribute
     {
         public NativeCppClassAttribute() { }
     }
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple=true)]
+    public sealed class CppInlineNamespaceAttribute : Attribute
+    {
+        public CppInlineNamespaceAttribute(string dottedName) {}
+    }
     [System.AttributeUsageAttribute(System.AttributeTargets.Class | System.AttributeTargets.Enum | System.AttributeTargets.Interface | System.AttributeTargets.Struct, AllowMultiple=true, Inherited=false)]
     public sealed partial class RequiredAttributeAttribute : System.Attribute
     {

src/libraries/System.Runtime.CompilerServices.VisualC/src/System/Runtime/CompilerServices/Attributes.cs

@@ -121,6 +121,12 @@ public sealed class NativeCppClassAttribute : Attribute
         public NativeCppClassAttribute() { }
     }
 
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple=true)]
+    public sealed class CppInlineNamespaceAttribute : Attribute
+    {
+        public CppInlineNamespaceAttribute(string dottedName) {}
+    }
+
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Interface, AllowMultiple = true, Inherited = false)]
     public sealed class RequiredAttributeAttribute : Attribute
     {