Save memory by re-using empty TagInfo (and subclasses) arrays.

Arrays of TagInfo (and subclasses) often tend to be empty, as many
members simply won't have any of the relevant tags.
Re-using empty arrays saves potentially a dozen or more megabytes
for large projects.

Also some minor performance tweaks the Javadoc comment parsing.

Change-Id: Iffaddd3b59a4f8183f11efd6e2680aad95633ade
diff --git a/src/com/google/doclava/SampleTagInfo.java b/src/com/google/doclava/SampleTagInfo.java
index a69a3f3..66eb9ac 100644
--- a/src/com/google/doclava/SampleTagInfo.java
+++ b/src/com/google/doclava/SampleTagInfo.java
@@ -44,6 +44,12 @@
  * samples/ApiDemos/src/com/google/app/Notification1.java Bleh}
  */
 public class SampleTagInfo extends TagInfo {
+  public static final SampleTagInfo[] EMPTY_ARRAY = new SampleTagInfo[0];
+
+  public static SampleTagInfo[] getArray(int size) {
+      return size == 0 ? EMPTY_ARRAY : new SampleTagInfo[size];
+  }
+
   static final int STATE_BEGIN = 0;
   static final int STATE_MATCHING = 1;