Frontend: Add CodeGenAction support for handling LLVM IR.

and 'llvm-dis' is:  $ clang -cc1 -emit-llvm    FOO.bc -o -
and 'opt' is, e.g.:
  $ clang -cc1 -emit-llvm -O3 -o FOO.opt.ll FOO.ll
and 'llc' is, e.g.:
  $ clang -cc1 -S -o - FOO.ll

The nice thing about using the backend tools this way is that they are guaranteed to exactly match how the compiler generates code (for example, setting the same backend options).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@105583 91177308-0d34-0410-b5e6-96231b3b80d8