clang-cl: Expose the -flto option

We could hook up /GL as an alias for -flto, but that might be
confusing, as clang-cl in that mode would not be drop-in compatible
with cl.exe /GL, as it requires the linker to be lld.

Exposing -flto seems like a less confusing way to expose this
functionality.

llvm-svn: 283255