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/GettingStarted.rst b/llvm/docs/GettingStarted.rst
index 524a3c2..742c7b9 100644
--- a/llvm/docs/GettingStarted.rst
+++ b/llvm/docs/GettingStarted.rst
@@ -240,7 +240,7 @@
 * Clang 3.1
 * Apple Clang 3.1
 * GCC 4.8
-* Visual Studio 2015 (Update 3)
+* Visual Studio 2017
 
 Anything older than these toolchains *may* work, but will require forcing the
 build system with a special option and is not really a supported host platform.
@@ -275,7 +275,7 @@
 This section mostly applies to Linux and older BSDs. On macOS, you should
 have a sufficiently modern Xcode, or you will likely need to upgrade until you
 do. Windows does not have a "system compiler", so you must install either Visual
-Studio 2015 or a recent version of mingw64. FreeBSD 10.0 and newer have a modern
+Studio 2017 or a recent version of mingw64. FreeBSD 10.0 and newer have a modern
 Clang as the system compiler.
 
 However, some Linux distributions and some other or older BSDs sometimes have