Take release level into account when check for java9

Currently in SearchableEnvironment we only check if the project level is
larger java9plus but when constructing a release specific
SearchableEnvironment this needs to take the release option into account
instead.

Fix #4399

Author: laeubi

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/SearchableEnvironment.java

@@ -115,7 +115,7 @@ public SearchableEnvironment(JavaProject project, org.eclipse.jdt.core.ICompilat
 			|| !JavaCore.IGNORE.equals(project.getOption(JavaCore.COMPILER_PB_DISCOURAGED_REFERENCE, true));
 		this.workingCopies = workingCopies;
 		this.nameLookup = project.newNameLookup(workingCopies, excludeTestCode);
-		boolean java9plus = JavaCore.callReadOnly(() -> CompilerOptions
+		boolean java9plus = release >=JavaProject.FIRST_MULTI_RELEASE || JavaCore.callReadOnly(() -> CompilerOptions
 				.versionToJdkLevel(project.getOption(JavaCore.COMPILER_COMPLIANCE, true)) >= ClassFileConstants.JDK9);
 		if (java9plus) {
 			this.knownModuleLocations = new HashMap<>();