IR: Add missing API to specialized metadata nodes

Add the final bits of API that `DIBuilder` needs before the new nodes
can be moved into place.

  - Add `MDType::clone()` and `MDType::setFlags()` to support
    `DIBuilder::createTypeWithFlags()`.
  - Add `MDBasicType::get()` overload that just requires a tag and a
    name, as a convenience for `DIBuilder::createUnspecifiedType()`.
  - Add `MDLocalVariable::withInline()` and
    `MDLocalVariable::withoutInline()` to support
    `llvm::createInlinedVariable()` and
    `llvm::cleanseInlinedVariable()`.

(Somehow these got lost inside the "move into place" patch I'm about to
commit -- better to commit separately!)

llvm-svn: 231079