Fix unbounded stackalloc in System.IO.FileSystem (dotnet/corefx#28004)

Commit migrated from https://github.com/dotnet/corefx/commit/bf36018813d2724e5bc9140d72e0d8bf92f2b621

src/libraries/System.IO.FileSystem/src/System/IO/Enumeration/FileSystemName.cs

@@ -34,7 +34,7 @@ public static string TranslateWin32Expression(string expression)
                 return "*";
 
             bool modified = false;
-            Span<char> stackSpace = stackalloc char[expression.Length];
+            Span<char> stackSpace = stackalloc char[32];
             ValueStringBuilder sb = new ValueStringBuilder(stackSpace);
             int length = expression.Length;
             for (int i = 0; i < length; i++)

src/libraries/System.IO.FileSystem/tests/Enumeration/FileSystemNameTests.netcoreapp.cs

@@ -143,5 +143,12 @@ public void TranslateExpression(string expression, string expected)
         {
             Assert.Equal(expected, FileSystemName.TranslateWin32Expression(expression));
         }
+
+        [Fact]
+        public void TranslateVeryLongExpression()
+        {
+            string longString = new string('a', 10_000_000);
+            Assert.Equal(longString, FileSystemName.TranslateWin32Expression(longString));
+        }
     }
 }