[CMake] Update to retiring CMake 3.4.3

[beanz]

Summary

This is as per the discussions on developer lists:

http://lists.llvm.org/pipermail/llvm-dev/2016-April/098780.html
http://lists.llvm.org/pipermail/llvm-dev/2016-May/100058.html

llvm/trunk/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
-cmake_minimum_required(VERSION 2.8.12.2)
+cmake_minimum_required(VERSION 3.4.3)
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
   set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)")
 endif()
 
 if(POLICY CMP0022)
   cmake_policy(SET CMP0022 NEW) # automatic when 2.8.12 is required

llvm/trunk/docs/CMake.rst

 now reliably use CMake to develop their own LLVM-based projects against an
 installed version of LLVM regardless of how it was built.
 
 Here is a simple example of a CMakeLists.txt file that imports the LLVM libraries
 and uses them to build a simple application ``simple-tool``.
 
 .. code-block:: cmake
 
-  cmake_minimum_required(VERSION 2.8.8)
+  cmake_minimum_required(VERSION 3.4.3)
   project(SimpleProject)
 
   find_package(LLVM REQUIRED CONFIG)
 
   message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
   message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 
   # Set your project compile flags.

llvm/trunk/docs/GettingStarted.rst

#. Get the Test Suite Source Code **[Optional]**
    * ``svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite``
 
 #. Configure and build LLVM and Clang:
 
    *Warning:* Make sure you've checked out *all of* the source code 
    before trying to configure with cmake.  cmake does not pickup newly
    added source directories in incremental builds. 
 
-   The build uses `CMake <CMake.html>`_.
-   Although the build is known to work with CMake >= 2.8.8, we recommend CMake
-   >= v3.2, especially if you're generating Ninja build files.
+   The build uses `CMake <CMake.html>`_. LLVM requires CMake 3.4.3 to build. It
+   is generally recommended to use a recent CMake, especially if you're
+   generating Ninja build files. This is because the CMake project is constantly
+   improving the quality of the generators, and the Ninja generator gets a lot
+   of attention.
 
    * ``cd where you want to build llvm``
    * ``mkdir build``
    * ``cd build``
    * ``cmake -G <generator> [options] <path to llvm sources>``
 
      Some common generators are: