Fix performance problem in long-running SectionMemoryManagers

Summary:
Without this patch, the memory manager would call `mprotect` on every memory
region it ever allocated whenever it wanted to finalize memory (i.e. not just
the ones it just allocated). This caused terrible performance problems for
long running memory managers. In one particular compile heavy julia benchmark,
we were spending 50% of time in `mprotect` if running under MCJIT.

Fix this by splitting allocated memory blocks into those on which memory
permissions have been set and those on which they haven't and only running
`mprotect` on the latter.

Reviewers: lhames

Subscribers: reames, llvm-commits

Differential Revision: http://reviews.llvm.org/D13156

llvm-svn: 248981