DebugInfo: Drop dead code for loose DIDescriptor construction API

Delete `DIDescriptor::is*()` and the various constructors from `MDNode*`
in `DIDescriptor` subclasses.

If this just broke your out-of-tree code, you need to make updates along
the lines of r234255, r234256, r234257 and r234258:

  - Generally, `DIX().isX()` => `isa<MDX>()`.  So, `D.isCompileUnit()`
    should just be `isa<MDCompileUnit>(D)`, modulo checks for null.
      - Exception: `DILexicalBlock` => `MDLexicalBlockBase`.
      - Exception: `DIDerivedType` => `MDDerivedTypeBase`.
      - Exception: `DICompositeType` => `MDCompositeTypeBase`.
      - Exception: `DIVariable` => `MDLocalVariable`.
  - Note that (e.g.) `DICompileUnit` has an implicit constructor from
    `MDCompileUnit*`.

llvm-svn: 234263