Ensure DataSource can load database driver

Update DataSource conditional to ensure that the driver class
can actually be loaded by the DataSource. This fixes an issue when
deploying a classic WAR where `org.apache.tomcat.jdbc.pool.DataSource`
is found the parent classloader but the database driver cannot be loaded
because is included as a local `/lib` dependency.

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration.java

@@ -183,7 +183,7 @@ public class DataSourceAutoConfiguration implements EnvironmentAware {
 			}
 
 			String driverClassName = getDriverClassName(context.getEnvironment(),
-					context.getClassLoader());
+					getDataSourceClassLoader(context));
 			if (driverClassName == null) {
 				return Outcome.noMatch("no database driver");
 			}
@@ -200,6 +200,21 @@ public class DataSourceAutoConfiguration implements EnvironmentAware {
 			return Outcome.noMatch("missing database driver " + driverClassName);
 		}
 
+		/**
+		 * Returns the class loader for the {@link DataSource} class. Used to ensure that
+		 * the driver class can actually be loaded by the data source.
+		 */
+		private ClassLoader getDataSourceClassLoader(ConditionContext context) {
+			try {
+				Class<?> dataSourceClass = ClassUtils.forName(getDataSourceClassName(),
+						context.getClassLoader());
+				return dataSourceClass.getClassLoader();
+			}
+			catch (ClassNotFoundException ex) {
+				throw new IllegalStateException(ex);
+			}
+		}
+
 		private String getDriverClassName(Environment environment, ClassLoader classLoader) {
 			String driverClassName = environment == null ? null : environment
 					.getProperty("spring.database.driverClassName");