ASTParser.createAST looses ability to resolve some bindings

Having root unit set in the lookupEnvironment (when focusing on a single
unit) can be necessary for
further binding resolution.

Fixes eclipse-jdt#1915

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/dom/ASTConverter15Test.java

@@ -7867,13 +7867,13 @@ public void test0238() throws JavaModelException {
 	 * https://bugs.eclipse.org/bugs/show_bug.cgi?id=173338
 	 */
 	public void test0238_2() throws JavaModelException {
-		this.workingCopy = getWorkingCopy("/Converter15/src/test0238/X.java", true/*resolve*/);
-		String contents =
-			"package test0238;\n" +
-			"public class X extends A {\n" +
-			"}";
+		this.workingCopy = getWorkingCopy("/Converter15/src/test0238/X.java",
+			"""
+			package test0238;
+			public class X extends A {
+			}""", true/*resolve*/);
 		ASTNode node = buildAST(
-				contents,
+				this.workingCopy.getSource(),
 				this.workingCopy,
 				false,
 				false,

org.eclipse.jdt.core/dom/org/eclipse/jdt/core/dom/CompilationUnitResolver.java

@@ -1266,6 +1266,9 @@ private CompilationUnitDeclaration resolve(
 				if (unit == null) {
 					unit = this.unitsToProcess[0]; // fall back to old behavior
 				}
+				if (this.totalUnits == 1 && this.lookupEnvironment.unitBeingCompleted == null) {
+					this.lookupEnvironment.unitBeingCompleted = unit;
+				}
 			} else {
 				// initial type binding creation
 				this.lookupEnvironment.buildTypeBindings(unit, null /*no access restriction*/);