[foreign-jextract] RFR 8236434: Initial jextract API implementation

[sundararajan.athijegannathan at oracle.com]

This review is response to 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-December/006905.html

I did quick review. Also checked build/test on Mac OS. All fine.

* File: ClangUtils (preexisting code). This code caches stuff as static 
fields and so won't allow proper "instancing" of clang artifacts across 
jextract tasks for example. Besides plan is to handle Atomic by 
modification of libclang, right? Do we want this in the new code?

* File: src/jdk.incubator.jextract/share/classes/module-info.java

copyright is missing

-Sundar