[Support/MemoryBuffer] Introduce a boolean parameter (false by default) 'IsVolatile' for the open file functions.

This provides a hint that the file may be changing often so mmap is avoided.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208007 91177308-0d34-0410-b5e6-96231b3b80d8