ADT: introduce isWindowsItaniumEnvironment

Add an isWindowsItaniumEnvironment function to Triple to mirror the other
Windows environments.  This is simply a utility function to check if we are
targeting windows-itanium rather than windows-msvc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210383 91177308-0d34-0410-b5e6-96231b3b80d8