Re-did some of the text wrapping (sorry).

Re-did some of the text wrapping (sorry).
Made the "Getting Started Quickly" material its own section.  I think this makes
the document easier to read.
Added bars after every major section heading (to distinguish them more easily from
sub-section headings).
Renamed C front end to GCC front end, as we now support C and C++.
Updated material to reflect the new autoconf-style object root.
Added material about the llvm/runtime directory and the fact that you need to
install the GCC front end before building LLVM (before, it was optional).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9003 91177308-0d34-0410-b5e6-96231b3b80d8