[SectionMemoryManager] Don't just drop the RO free list

[SectionMemoryManager] Don't just drop the RO free list

In r255760, I optimized the SectionMemoryManager to make better use
of virtual memory on platforms where the allocation granularity was
bigger than the protection granularity. As part of this, fixing up
the free list became more complicated and was moved into
`applyMemoryGroupPermissions`. Unfortunately, I forgot to actually
remove the call that drops the free list for RO memory (I did
remove the corresponding one for RX memory), defeating the whole
optimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257293 91177308-0d34-0410-b5e6-96231b3b80d8