Recommit "MemoryBuffer: Add a missing error-check to getOpenFileImpl"

This recommits r368977, which was reverted in r369027 due to test
failures in lldb. The cause of this was different behavior of
readNativeFileSlice on windows and unix. These have been addressed in
r369269.

The original commit message was:
In case the function was called with a desired read size *and* the file
was not an "mmap()" candidate, the function was falling back to a
"pread()", but it was failing to check the result of that system call.
This meant that the function would return "success" even though the read
operation failed, and it returned a buffer full of uninitialized memory.

Reviewers: rnk, dblaikie

Subscribers: kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66224

llvm-svn: 369370