Merge lib/Support/WindowsError.cpp into ib/Support/ErrorHandling.cpp.

The OSX ranlib warns on files with no symbols, and lib/Support/WindowsError.cpp
was empty when building on non-windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211118 91177308-0d34-0410-b5e6-96231b3b80d8