Move UTF functions into namespace llvm.

Summary:
This lets people link against LLVM and their own version of the UTF
library.

I determined this only affects llvm, clang, lld, and lldb by running

$ git grep -wl 'UTF[0-9]\+\|\bConvertUTF\bisLegalUTF\|getNumBytesFor' | cut -f 1 -d '/' | sort | uniq
  clang
  lld
  lldb
  llvm

Tested with

  ninja lldb
  ninja check-clang check-llvm check-lld

(ninja check-lldb doesn't complete for me with or without this patch.)

Reviewers: rnk

Subscribers: klimek, beanz, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D24996

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282822 91177308-0d34-0410-b5e6-96231b3b80d8