[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