Split getOpenFile into getOpenFile and getOpenFileSlice.

The main observation is that we never need both the filesize and the map size.
When mapping a slice of a file, it doesn't make sense to request a null
terminator and that would be the only case where the filesize would be used.

There are other cleanups that should be done in this area:

* A client should not have to pass the size (even an explicit -1) to say if
  it wants a null terminator or not, so we should probably swap the argument
  order.
* The default should be to not require a null terminator. Very few clients
  require this, but many end up asking for it just because it is the default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186984 91177308-0d34-0410-b5e6-96231b3b80d8
Debug Info: code cleanup, use getNodeField if possible.

Use getNodeField to access a field as a MDNode.
No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186985 91177308-0d34-0410-b5e6-96231b3b80d8
Refactor the unit test for MemoryBuffer::getOpenFileSlice

Run in two different modes: with and without reopening the temporary file
between creating it and mapping it with MemoryBuffer.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186986 91177308-0d34-0410-b5e6-96231b3b80d8
Expand the comment on getOpenFileSlice a bit for more details and consistency


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186987 91177308-0d34-0410-b5e6-96231b3b80d8