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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365452 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/CheckCompilerVersion.cmake

@@ -10,7 +10,10 @@ set(CLANG_MIN 3.1)
 set(CLANG_SOFT_ERROR 3.5)
 set(APPLECLANG_MIN 3.1)
 set(APPLECLANG_SOFT_ERROR 6.0)
-set(MSVC_MIN 19.00.24213.1)
+
+# https://en.wikipedia.org/wiki/Microsoft_Visual_C#Internal_version_numbering
+# _MSC_VER == 1910 MSVC++ 14.1 (Visual Studio 2017 version 15.0)
+set(MSVC_MIN 19.1)
 set(MSVC_SOFT_ERROR 19.1)
 
 # Map the above GCC versions to dates: https://gcc.gnu.org/develop.html#timeline

docs/GettingStarted.rst

@@ -240,7 +240,7 @@ with the following compilers, though this will change in the near future:
 * 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 @@ Getting a Modern Host C++ Toolchain
 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

docs/GettingStartedVS.rst

@@ -39,13 +39,13 @@ and software you will need.
 
 Hardware
 --------
-Any system that can adequately run Visual Studio 2015 is fine. The LLVM
+Any system that can adequately run Visual Studio 2017 is fine. The LLVM
 source tree and object files, libraries and executables will consume
 approximately 3GB.
 
 Software
 --------
-You will need Visual Studio 2015 or higher, with the latest Update installed.
+You will need Visual Studio 2017 or higher, with the latest Update installed.
 
 You will also need the `CMake <http://www.cmake.org/>`_ build system since it
 generates the project files you will use to build with.

docs/ReleaseNotes.rst

@@ -76,6 +76,12 @@ Changes to the LLVM IR
   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
 --------------------------