Fix insert of annotation to not change indent of following line (#3566)

- add new method computeIndentInSpaces() to ASTRewriteFormatter
- modify ASTRewriteAnalyzer.rewriteModifiers2() to add spaces as
  needed to separator of added annotation to match current indent
- needed for eclipse-jdt/eclipse.jdt.ui#1940

org.eclipse.jdt.core/dom/org/eclipse/jdt/internal/core/dom/rewrite/ASTRewriteAnalyzer.java

@@ -1733,6 +1733,9 @@ else if (parent instanceof Block || parent instanceof TryStatement || parent ins
 				String separator;
 				if (lastChild.getNewValue() instanceof Annotation) {
 					separator= formatterPrefix.getPrefix(getIndent(pos));
+					for (int i= 0; i < this.formatter.computeIndentInSpaces(getIndentOfLine(pos)) % this.formatter.getTabWidth(); ++i) {
+						separator += " "; //$NON-NLS-1$
+					}
 				} else {
 					separator= String.valueOf(' ');
 				}

org.eclipse.jdt.core/dom/org/eclipse/jdt/internal/core/dom/rewrite/ASTRewriteFormatter.java

@@ -212,6 +212,10 @@ public int computeIndentUnits(String line) {
 		return IndentManipulation.measureIndentUnits(line, this.tabWidth, this.indentWidth);
 	}
 
+	public int computeIndentInSpaces(String line) {
+		return IndentManipulation.measureIndentInSpaces(line, this.tabWidth);
+	}
+
 	/**
 	 * Evaluates the edit on the given string.
 	 * @param string The string to format