Retire VS2015 Support

As proposed here: https://lists.llvm.org/pipermail/llvm-dev/2019-June/133147.html

This patch raises the minimum supported version to build LLVM/Clang to Visual Studio 2017.

Differential Revision: https://reviews.llvm.org/D64326

llvm-svn: 365452
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index 7942718..4ad4c94 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -76,6 +76,12 @@
   pointee type. In the next release we intend to make this parameter
   mandatory in preparation for opaque pointer types.
 
+Changes to building LLVM
+------------------------
+
+* Building LLVM with Visual Studio now requires version 2017 or later.
+
+
 Changes to the ARM Backend
 --------------------------