[examples] Fix SectionMemoryManager deconstruction error with MSVC.

This commit fixes an order-of-initialization issue: If the default mmapper
object is destroyed while some global SectionMemoryManager is still using it
then calls to the mapper from ~SectionMemoryManager will fail. This issue was
causing failures when running the LLVM Kaleidoscope examples on windows.

Switching to a ManagedStatic solves the initialization order issue.

Patch by Justice Adams. Thanks Justice!

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D107087