Add a release note about raising the minimum Windows version for the next major release

[gbedwell]

Summary

As we're planning on raising the minimum Windows version to Windows 7 for LLVM 3.8 as per this discussion (there have been no objections I'm aware of so far even after advertising the discussion in LLVM weekly):
http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-July/087993.html
I'd like to add a release note to mention this fact. I've put it under the list of changes in this release although it's actually a notification of a future change. I wasn't sure whether to create a new section or not. I guess we'll need a matching Clang release note too so I'll look at that now.

[rnk]

Sounds good. Make sure to commit this to the release_37 branch, not trunk.

[aaron.ballman]

LGTM, and yes, we should have this in the Clang release notes section as well.

[hans]

I went ahead and committed this in r246071.

[gbedwell]

In D12367#233511, @hans wrote:

I went ahead and committed this in r246071.

Thanks!

llvm/branches/release_37/docs/ReleaseNotes.rst

* LLVM IR and APIs are in a period of transition to aid in the removal of
   migration will be.
 
 * Argument-less ``TargetMachine::getSubtarget`` and
   ``TargetMachine::getSubtargetImpl`` have been removed from the tree. Updating
   out of tree ports is as simple as implementing a non-virtual version in the
   target, but implementing full ``Function`` based ``TargetSubtargetInfo``
   support is recommended.
 
+* This is expected to be the last major release of LLVM that supports being
+  run on Windows XP and Windows Vista.  For the next major release the minimum
+  Windows version requirement will be Windows 7.
+
 Changes to the MIPS Target
 --------------------------
 
 During this release the MIPS target has:
 
 * Added support for MIPS32R3, MIPS32R5, MIPS32R3, MIPS32R5, and microMIPS32.
 
 * Added support for dynamic stack realignment. This is of particular importance