Avoid constructing GlobalExtensions only to find out it is empty.

Summary:
GlobalExtensions is dereferenced twice, once for iteration and then a check if it is empty.
As a ManagedStatic this dereference forces it's construction which is unnecessary.

Reviewers: efriedma, davide, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: chapuni, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D33381

llvm-svn: 307229