[Support] Fix lifetime of file descriptors when using MemoryBuffer.

Clients of MemoryBuffer::getOpenFile expect it not to take ownership of the file
descriptor passed in. So don't.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176995 91177308-0d34-0410-b5e6-96231b3b80d8