SPARK-1494 Don't initialize classes loaded by MIMA excludes, attempt 2

[WIP]

Looks like scala reflection was invoking the static initializer:
```
...
	at org.apache.spark.sql.test.TestSQLContext$.<init>(TestSQLContext.scala:25)
	at org.apache.spark.sql.test.TestSQLContext$.<clinit>(TestSQLContext.scala)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:270)
	at scala.reflect.runtime.JavaMirrors$JavaMirror.javaClass(JavaMirrors.scala:500)
	at scala.reflect.runtime.JavaMirrors$JavaMirror.tryJavaClass(JavaMirrors.scala:505)
	at scala.reflect.runtime.SymbolLoaders$PackageScope.lookupEntry(SymbolLoaders.scala:109)
...
```

Need to make sure that this doesn't change the exclusion semantics before merging.

Author: Michael Armbrust <michael@databricks.com>

Closes #526 from marmbrus/mima and squashes the following commits:

8168dea [Michael Armbrust] Spurious change
afba262 [Michael Armbrust] Prevent Scala reflection from running static class initializer.

tools/src/main/scala/org/apache/spark/tools/GenerateMIMAIgnore.scala

@@ -50,7 +50,7 @@ object GenerateMIMAIgnore {
            is a module or class. */
 
         val privateAsClass = mirror
-          .staticClass(className)
+          .classSymbol(Class.forName(className, false, classLoader))
           .privateWithin
           .fullName
           .startsWith(packageName)