[adt] Twine(nullptr) derefs the nullptr. Add a deleted Twine(std::nullptr_t)

Summary:
nullptr can implicitly convert to Twine as Twine(nullptr) in which case it
resolves to Twine(const char *). This constructor derefs the pointer and
therefore doesn't work. Add a Twine(std::nullptr_t) = delete to make it a
compile time error.

It turns out that in-tree usage of Twine(nullptr) is confined to a single
private method in IRBuilder where foldConstant(... const Twine &Name = nullptr)
and this method is only ever called with an explicit Name argument as making it
a mandatory argument doesn't cause compile-time or run-time errors.

Reviewers: jyknight

Reviewed By: jyknight

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56870

llvm-svn: 351572