RFR: JDK-8076583: move jdk.Exported from langtools to jdk

[Jonathan Gibbons]

Sorry for the relatively wide distribution.

JDK-8076583 is a conceptually simple cleanup, to move the source file 
for the jdk.Exported class from the langtools repo (where it is a 
singleton outlier) to the jdk repo (alongside most of the rest of the 
classes in the jdk package hierarchy). The class was originally placed 
in the langtools repo for bootstrapping reasons that no longer apply.

As a result of moving the source file, references to java.base in a 
number of places in the langtools repo can be cleaned up.

@Build folk:
There is a trivial change to a makefile in the langtools repo.

@Core-libs folk:
The source file gets moved into the jdk/ repo.

@Compiler-dev folk:
We can remove references to the java.base folder from the private Ant 
build, and from IDE support files.

JBS: https://bugs.openjdk.java.net/browse/JDK-8076583
Webrev: http://cr.openjdk.java.net/~jjg/8076583/webrev.00/

-- Jon