Make triple-slash comment classification more restrictive

It was overly permissive and ended up making a mess of C#-style
comments:

`/// <summary>Text</summary>`

Now it checks the element name.  Attribute names remain unchecked.

src/services/classifier.ts

@@ -771,6 +771,14 @@ namespace ts {
                 return false;
             }
 
+            // Limiting classification to exactly the elements and attributes
+            // defined in `ts.commentPragmas` would be excessive, but we can avoid
+            // some obvious false positives (e.g. in XML-like doc comments) by
+            // checking the element name.
+            if (!match[3] || !(commentPragmas as any)[match[3]]) {
+                return false;
+            }
+
             let pos = start;
 
             pushCommentRange(pos, match[1].length); // ///
@@ -779,10 +787,6 @@ namespace ts {
             pushClassification(pos, match[2].length, ClassificationType.punctuation); // <
             pos += match[2].length;
 
-            if (!match[3]) {
-                return true;
-            }
-
             pushClassification(pos, match[3].length, ClassificationType.jsxSelfClosingTagName); // element name
             pos += match[3].length;
 

tests/cases/fourslash/syntacticClassificationsTripleSlash17.ts

@@ -0,0 +1,7 @@
+/// <reference path="fourslash.ts"/>
+
+//// /// <summary>Text</summary>
+
+var c = classification;
+verify.syntacticClassificationsAre(
+    c.comment("/// <summary>Text</summary>"));

tests/cases/fourslash/syntacticClassificationsTripleSlash18.ts

@@ -0,0 +1,7 @@
+/// <reference path="fourslash.ts"/>
+
+//// /// <reference>Text</reference>
+
+var c = classification;
+verify.syntacticClassificationsAre(
+    c.comment("/// <reference>Text</reference>"));

tests/cases/fourslash/syntacticClassificationsTripleSlash4.ts

@@ -4,5 +4,4 @@
 
 var c = classification;
 verify.syntacticClassificationsAre(
-    c.comment("/// "),
-    c.punctuation("<"));
+    c.comment("/// <")); // Don't classify until we recognize the element name