Implement __attribute__((internal_linkage)).

Implement __attribute__((internal_linkage)).

The attrubite is applicable to functions and variables and changes
the linkage of the subject to internal.

This is the same functionality as C-style "static", but applicable to
class methods; and the same as anonymouns namespaces, but can apply
to individual methods of a class.

Following the proposal in
http://lists.llvm.org/pipermail/cfe-dev/2015-October/045580.html

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252648 91177308-0d34-0410-b5e6-96231b3b80d8