Add Lazy<T>(T value) constructor (#6304)

Fix dotnet/corefx#2578

Author: Peter Jas

src/mscorlib/model.xml

@@ -3134,6 +3134,7 @@
     </Type>
     <Type Name="System.Lazy&lt;T&gt;">
       <Member Name="#ctor"/>
+      <Member Name="#ctor(T)"/>
       <Member Name="#ctor(System.Func&lt;T&gt;)"/>
       <Member Name="#ctor(System.Boolean)"/>
       <Member Name="#ctor(System.Threading.LazyThreadSafetyMode)"/>

src/mscorlib/src/System/Lazy.cs

@@ -125,6 +125,19 @@ public Lazy()
         {
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="T:System.Threading.Lazy{T}"/> class that
+        /// uses a pre-initialized specified value.
+        /// </summary>
+        /// <remarks>
+        /// An instance created with this constructor should be usable by multiple threads
+        //  concurrently.
+        /// </remarks>
+        public Lazy(T value)
+        {
+            m_boxed = new Boxed(value);
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="T:System.Threading.Lazy{T}"/> class that uses a
         /// specified initialization function.