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