[docs] Clean up some of the required software to not mention irrelevant
bits of software and to use a modern GCC version.

The Subversion bit was weird anyways -- it has nothing to do with
compiling LLVM. Also, there are many other ways to get at the trunk
source (git, git-svn, etc).

The TeXinfo thing... I have no idea about. But you can get a working
LLVM w/o it pretty easily. If man pages or something are missing, that
hardly seems like a problem. If folks really want this back, let me
know, but it seems mostly like a distraction.

I'd still like to separate this into:
- Required software to compile.
- Optional software to compile.
- Required software for certain *contributor* activities (like
  regenerating configure scripts).

Also we need to mention that there are multiple options for build
systems, and the differences.

Also we should mention Windows.

Also probably other stuff I'm forgetting.

I'm wondering if this whole thing needs to be shot in the head and we
should just start a new, simpler getting started that doesn't have so
many years of accumulated stuff that is no longer relevant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202373 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Actually spell out the new version requirements for the host C++
toolchain of LLVM. These are already being enforced by the build system
and have been discussed quite a few times on the lists, but
documentation is important. =]

Also, garbage collect the majority of the information about broken host
GCC toolchains. These aren't really relevant any more as they're all
older than the minimum requirement. I've left a few notes about
compilers one step older than the current requirement as these compilers
are at least conceivable to use, and it's better to preserve this kind
of hard-won institutional knowledge.

The next step will be some specific docs on how to set up a sufficiently
modern host toolchain if your system doesn't come with one. But that'll
be tomorrow. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202375 91177308-0d34-0410-b5e6-96231b3b80d8