[docs] End users should see just "Clang"

Half the internet has wildly incorrect ideas about what LLVM is (in
particular, many get the impression that LLVM is some magical
cross-platform runtime), so saying "LLVM" near "cross-compilation" in a
user-facing doc might provoke confusion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190338 91177308-0d34-0410-b5e6-96231b3b80d8