clang-cl: Expose the -flto option

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283255 91177308-0d34-0410-b5e6-96231b3b80d8