Fixed two places where we needed to force completion of a type

(without complaining if it fails) to get proper semantics: reference
binding with a derived-to-base conversion and the enumeration of
constructors for user-defined conversions. There are probably more
cases to fix, but my prior attempt at statically ensuring that
complete-type checking always happens failed. Perhaps I'll try again.

With this change, Clang can parse include/llvm/*.h!

llvm-svn: 86129