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

tools/llvm-ar/llvm-ar.cpp

@@ -770,9 +770,9 @@ static void performWriteOperation(ArchiveOperation Operation,
       failIfError(sys::fs::status(FD, Status), FileName);
 
       OwningPtr<MemoryBuffer> File;
-      failIfError(
-          MemoryBuffer::getOpenFile(FD, FileName, File, Status.getSize()),
-          FileName);
+      failIfError(MemoryBuffer::getOpenFile(FD, FileName, File,
+                                            Status.getSize(), false),
+                  FileName);
 
       StringRef Name = sys::path::filename(FileName);
       if (Name.size() < 16)