MC: add and use an accessor for WinCFI

This adds a utility method to access the WinCFI information in bulk and uses
that to iterate rather than requesting the count and individually iterating
them.  This is in preparation for restructuring WinCFI handling to enable more
clear sharing across architectures to enable unwind information emission for
Windows on ARM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212683 91177308-0d34-0410-b5e6-96231b3b80d8
MC: modernise for loop

Convert a for loop to range bsaed form.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212684 91177308-0d34-0410-b5e6-96231b3b80d8