Add CodeGen support for adding cpu attributes on functions based on

the target-cpu, if different from the triple's cpu, and
target-features as they're written that are passed down from the
driver.

Together with LLVM r232885 this should allow the LTO'ing of binaries
that contain modules compiled with different code generation options
on a subset of architectures with full backend support (x86, powerpc,
aarch64).

llvm-svn: 232888