Add dummy classes for xtext @pure and ToStringBuilder to prevent

ClassNotFoundExceptions while debugging.  See
eclipse-lsp4j/lsp4j#494 for context.

[Bug]

tlatools/org.lamport.tlatools/src/org/eclipse/xtext/xbase/lib/Pure.java

@@ -0,0 +1,16 @@
+package org.eclipse.xtext.xbase.lib;
+
+import static java.lang.annotation.ElementType.CONSTRUCTOR;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+@Documented
+@Retention(RUNTIME)
+@Target({ METHOD, CONSTRUCTOR })
+public @interface Pure {
+
+}

tlatools/org.lamport.tlatools/src/org/eclipse/xtext/xbase/lib/util/ToStringBuilder.java

@@ -0,0 +1,15 @@
+package org.eclipse.xtext.xbase.lib.util;
+
+public class ToStringBuilder {
+
+	public ToStringBuilder(Object instance) {
+	}
+
+	public ToStringBuilder add(Object value) {
+		return this;
+	}
+
+	public ToStringBuilder add(String fieldName, Object value) {
+		return this;
+	}
+}