Bump minimum toolchain version

Summary:
The RFC on moving past C++11 got good traction:
  http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html

This patch therefore bumps the toolchain versions according to our policy:
  llvm.org/docs/DeveloperPolicy.html#toolchain

Subscribers: mgorny, jkorous, dexonsmith, llvm-commits, mehdi_amini, jyknight, rsmith, chandlerc, smeenai, hans, reames, lattner, lhames, erichkeane

Differential Revision: https://reviews.llvm.org/D57264

llvm-svn: 353374
diff --git a/llvm/docs/GettingStarted.rst b/llvm/docs/GettingStarted.rst
index 126b842..f528bc6 100644
--- a/llvm/docs/GettingStarted.rst
+++ b/llvm/docs/GettingStarted.rst
@@ -170,7 +170,7 @@
 Package                                                     Version      Notes
 =========================================================== ============ ==========================================
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor
-`GCC <http://gcc.gnu.org/>`_                                >=4.8.0      C/C++ compiler\ :sup:`1`
+`GCC <http://gcc.gnu.org/>`_                                >=5.1.0      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=2.7        Automated test suite\ :sup:`2`
 `zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 =========================================================== ============ ==========================================
@@ -228,6 +228,15 @@
 standards<CodingStandards>`. To enforce this language version, we check the most
 popular host toolchains for specific minimum versions in our build systems:
 
+* Clang 3.5
+* Apple Clang 6.0
+* GCC 5.1
+* Visual Studio 2017
+
+The below versions currently soft-error as we transition to the new compiler
+versions listed above. The LLVM codebase is currently known to compile correctly
+with the following compilers, though this will change in the near future:
+
 * Clang 3.1
 * Apple Clang 3.1
 * GCC 4.8
@@ -283,33 +292,36 @@
 distribution on which users have struggled with the version requirements is
 Ubuntu Precise, 12.04 LTS. For this distribution, one easy option is to install
 the `toolchain testing PPA`_ and use it to install a modern GCC. There is
-a really nice discussions of this on the `ask ubuntu stack exchange`_. However,
-not all users can use PPAs and there are many other distributions, so it may be
-necessary (or just useful, if you're here you *are* doing compiler development
-after all) to build and install GCC from source. It is also quite easy to do
-these days.
+a really nice discussions of this on the `ask ubuntu stack exchange`_ and a
+`github gist`_ with updated commands. However, not all users can use PPAs and
+there are many other distributions, so it may be necessary (or just useful, if
+you're here you *are* doing compiler development after all) to build and install
+GCC from source. It is also quite easy to do these days.
 
 .. _toolchain testing PPA:
   https://launchpad.net/~ubuntu-toolchain-r/+archive/test
 .. _ask ubuntu stack exchange:
-  http://askubuntu.com/questions/271388/how-to-install-gcc-4-8-in-ubuntu-12-04-from-the-terminal
+  https://askubuntu.com/questions/466651/how-do-i-use-the-latest-gcc-on-ubuntu/581497#58149
+.. _github gist:
+  https://gist.github.com/application2000/73fd6f4bf1be6600a2cf9f56315a2d91
 
-Easy steps for installing GCC 4.8.2:
+Easy steps for installing GCC 5.1.0:
 
 .. code-block:: console
 
-  % wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2
-  % wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2.sig
+  % gcc_version=5.1.0
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2.sig
   % wget https://ftp.gnu.org/gnu/gnu-keyring.gpg
-  % signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-4.8.2.tar.bz2.sig`
+  % signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-${gcc_version}.tar.bz2.sig`
   % if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
-  % tar -xvjf gcc-4.8.2.tar.bz2
-  % cd gcc-4.8.2
+  % tar -xvjf gcc-${gcc_version}.tar.bz2
+  % cd gcc-${gcc_version}
   % ./contrib/download_prerequisites
   % cd ..
-  % mkdir gcc-4.8.2-build
-  % cd gcc-4.8.2-build
-  % $PWD/../gcc-4.8.2/configure --prefix=$HOME/toolchains --enable-languages=c,c++
+  % mkdir gcc-${gcc_version}-build
+  % cd gcc-${gcc_version}-build
+  % $PWD/../gcc-${gcc_version}/configure --prefix=$HOME/toolchains --enable-languages=c,c++
   % make -j$(nproc)
   % make install
 
@@ -317,7 +329,7 @@
 of this information from.
 
 .. _GCC wiki entry:
-  http://gcc.gnu.org/wiki/InstallingGCC
+  https://gcc.gnu.org/wiki/InstallingGCC
 
 Once you have a GCC toolchain, configure your build of LLVM to use the new
 toolchain for your host compiler and C++ standard library. Because the new