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