[IR] Don't allow DLL storage-class and local linkage

Disallow this meaningless combination. Doing so simplifies analysis
of LLVM code w.r.t t DLL storage-class, and prevents mistakes with
DLL storage class.

- Change the assembler to reject DLL storage class on symbols with
  local linkage.
- Change the bitcode reader to clear the DLL Storage class when the
  linkage is local for auto-upgrading
- Update LangRef.

There is an existing restriction on non-default visibility and local
linkage which this is modelled on.

Differential Review: https://reviews.llvm.org/D134784