Move the API and implementation of clang::driver::getARMCPUForMArch() to llvm::Triple::getARMCPUForArch().

Suggested by Eric Christopher.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212846 91177308-0d34-0410-b5e6-96231b3b80d8