Do not remove a target file in FileOutputBuffer::create().

FileOutputBuffer::create() attempts to remove a target file if the file
is a regular one, which results in an unexpected result in a failure
scenario.

If something goes wrong and the user of FileOutputBuffer decides to not
call commit(), it leaves nothing. An existing file is removed, and no
new file is created.

What we should do is to atomically replace an existing file with a new
file using rename(), so that it wouldn't remove an existing file without
creating a new one.

Differential Revision: https://reviews.llvm.org/D38283

llvm-svn: 314345