Update getting started docs

compiler-rt is optional. We often get email from users with compiler-rt
build errors who don't actually need compiler-rt. Marking it optional
should help them avoid those potential problems.

While I'm here, update a reference to the build directory and remove an
obsolete reference to llvm-gcc. Nobody today is under the impression
that Clang depends on GCC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265963 91177308-0d34-0410-b5e6-96231b3b80d8