Sema: Allow null names to be passed in to isAcceptableTagRedeclaration

It's possible for TagRedeclarations to involve decls without a name,
ie, anonymous enums. We hit some undefined behaviour if we bind these
null names to the reference here.

We never dereference the name, so it's harmless if it's null - make it
a pointer to allow that.

Fixes the Modules/submodules-merge-defs.cpp test under ubsan.

llvm-svn: 241963