build.gradle: fix checkstyle plugin failure

The `checkstyle` plugin of Gradle fails in JDK11.

The main reason is that, in JDK11, the `user.dir` cannot be reset by Gradle.[1]
So, checkstyle plugin is unable to locate the suppressions file correctly.
Also, the `FileContentHolder` module are deprecated[2].

Let's add ` config_loc` variable suggested by Gradle to solve the problem.[1]

[1] gradle/gradle#8286
[2] checkstyle/checkstyle#5187

build.gradle

@@ -28,6 +28,7 @@ repositories {
 
 checkstyle {
     toolVersion = '8.1'
+    configDir = file("$rootProject.projectDir/config/checkstyle")
 }
 
 jacocoTestReport {

config/checkstyle/checkstyle.xml

@@ -26,16 +26,13 @@
   </module>
 
   <module name="SuppressionFilter">
-    <property name="file" value="config/checkstyle/suppressions.xml"/>
+    <property name="file" value="${config_loc}/suppressions.xml"/>
   </module>
 
 
   <!-- All Java AST specific tests live under TreeWalker module. -->
   <module name="TreeWalker">
 
-    <!-- Required for SuppressionCommentFilter to work -->
-    <module name="FileContentsHolder"/>
-
     <!-- Required to allow exceptions in code style -->
     <module name="SuppressionCommentFilter">
       <property name="offCommentFormat" value="CHECKSTYLE.OFF\: ([\w\|]+)"/>