Fix build errors on some configurations

It's been reported
<http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180611/559616.html>
that template argument deduction for RetryAfterSignal fails if open is
not prefixed with "::".

This should help us build correctly on those platforms and explicitly
specifying the namespace is more correct anyway.

llvm-svn: 334403
diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index d8cc853..4428c2f 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -216,7 +216,7 @@
   // Read into Buffer until we hit EOF.
   do {
     Buffer.reserve(Buffer.size() + ChunkSize);
-    ReadBytes = sys::RetryAfterSignal(-1, read, FD, Buffer.end(), ChunkSize);
+    ReadBytes = sys::RetryAfterSignal(-1, ::read, FD, Buffer.end(), ChunkSize);
     if (ReadBytes == -1)
       return std::error_code(errno, std::generic_category());
     Buffer.set_size(Buffer.size() + ReadBytes);