Release Notes: document -f{,no-}integrate-as

Now that the integrated assembler is considered a first class feature of the
compiler and has a proper feature flag, document the change in the compiler
flags.  Ensure that we indicate that the legacy flags are still available, but,
encourage users to switch to the feature flags.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201964 91177308-0d34-0410-b5e6-96231b3b80d8