[SPARK-13640][SQL] Synchronize ScalaReflection.mirror method.

## What changes were proposed in this pull request?

`ScalaReflection.mirror` method should be synchronized when scala version is `2.10` because `universe.runtimeMirror` is not thread safe.

## How was this patch tested?

I added a test to check thread safety of `ScalaRefection.mirror` method in `ScalaReflectionSuite`, which will throw the following Exception in Scala `2.10` without this patch:

```
[info] - thread safety of mirror *** FAILED *** (49 milliseconds)
[info]   java.lang.UnsupportedOperationException: tail of empty list
[info]   at scala.collection.immutable.Nil$.tail(List.scala:339)
[info]   at scala.collection.immutable.Nil$.tail(List.scala:334)
[info]   at scala.reflect.internal.SymbolTable.popPhase(SymbolTable.scala:172)
[info]   at scala.reflect.internal.Symbols$Symbol.unsafeTypeParams(Symbols.scala:1477)
[info]   at scala.reflect.internal.Symbols$TypeSymbol.tpe(Symbols.scala:2777)
[info]   at scala.reflect.internal.Mirrors$RootsBase.init(Mirrors.scala:235)
[info]   at scala.reflect.runtime.JavaMirrors$class.createMirror(JavaMirrors.scala:34)
[info]   at scala.reflect.runtime.JavaMirrors$class.runtimeMirror(JavaMirrors.scala:61)
[info]   at scala.reflect.runtime.JavaUniverse.runtimeMirror(JavaUniverse.scala:12)
[info]   at scala.reflect.runtime.JavaUniverse.runtimeMirror(JavaUniverse.scala:12)
[info]   at org.apache.spark.sql.catalyst.ScalaReflection$.mirror(ScalaReflection.scala:36)
[info]   at org.apache.spark.sql.catalyst.ScalaReflectionSuite$$anonfun$12$$anonfun$apply$mcV$sp$1$$anonfun$apply$1$$anonfun$apply$2.apply(ScalaReflectionSuite.scala:256)
[info]   at org.apache.spark.sql.catalyst.ScalaReflectionSuite$$anonfun$12$$anonfun$apply$mcV$sp$1$$anonfun$apply$1$$anonfun$apply$2.apply(ScalaReflectionSuite.scala:252)
[info]   at scala.concurrent.impl.Future$PromiseCompletingRunnable.liftedTree1$1(Future.scala:24)
[info]   at scala.concurrent.impl.Future$PromiseCompletingRunnable.run(Future.scala:24)
[info]   at scala.concurrent.impl.ExecutionContextImpl$$anon$3.exec(ExecutionContextImpl.scala:107)
[info]   at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
[info]   at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
[info]   at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
[info]   at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
```

Notice that the test will pass when Scala version is `2.11`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #11487 from ueshin/issues/SPARK-13640.