Correctly initialise global blocks on Windows.

Summary:
Windows does not allow globals to be initialised to point to globals in
another DLL.  Exported globals may be referenced only from code.  Work
around this by creating an initialiser that runs in early library
initialisation and sets the isa pointer.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50436

llvm-svn: 339317