Frontend: Add CodeGenAction support for handling LLVM IR.

 - This magically enables using 'clang -cc1' as a replacement for most of 'llvm-as', 'llvm-dis', 'llc' and 'opt' functionality.

For example, 'llvm-as' is:
  $ clang -cc1 -emit-llvm-bc FOO.ll -o FOO.bc
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).

llvm-svn: 105583