[SectionMemoryManager] Make better use of virtual memory

Summary: On Windows, the allocation granularity can be significantly
larger than a page (64K), so with many small objects, just clearing
the FreeMem list rapidly leaks quite a bit of virtual memory space
(if not rss). Fix that by only removing those parts of the FreeMem
blocks that overlap pages for which we are applying memory permissions,
rather than dropping the FreeMem blocks entirely.

Reviewers: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15202

llvm-svn: 255760