Relax complete-type check for functions and function pointers to allow void...

Relax complete-type check for functions and function pointers to allow void return type.  This bug was exposed by Eli Friedman's commit to clang r188324.  Anywhere this version of clang ships, this libc++ fix must follow.  However this fix is compatible with previous clangs, and so this libc++ doesn't need to wait for this clang.

llvm-svn: 188413