Split openFileForRead into Windows and Unix versions.

This has some advantages:

* Lets us use native, utf16 windows functions.
* Easy to produce good errors on windows about trying to use a
directory when we want a file.
* Simplifies the unix version a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186511 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Unix/Path.inc

@@ -815,6 +815,15 @@ error_code unmap_file_pages(void *base, size_t size) {
   return error_code::success();
 }
 
+error_code openFileForRead(const Twine &Name, int &ResultFD) {
+  SmallString<128> Storage;
+  StringRef P = Name.toNullTerminatedStringRef(Storage);
+  while ((ResultFD = open(P.begin(), O_RDONLY)) < 0) {
+    if (errno != EINTR)
+      return error_code(errno, system_category());
+  }
+  return error_code::success();
+}
 
 } // end namespace fs
 } // end namespace sys