DataLayout: Be more verbose when diagnosing problems in pointer specs

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223903 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Move call to dropAllReferences() to MDNode subclasses

Don't call `dropAllReferences()` from `MDNode::~MDNode()`, call it
directly from `~MDNodeFwdDecl()` and `~GenericMDNode()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223904 91177308-0d34-0410-b5e6-96231b3b80d8
AsmParser: Don't allow null bytes in BB labels

Since Value objects can't have null bytes in their name, we shouldn't
allow them in the labels of basic blocks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223907 91177308-0d34-0410-b5e6-96231b3b80d8