Audit and finish the implementation of C++0x nullptr, fixing two

Audit and finish the implementation of C++0x nullptr, fixing two
minor issues along the way:
  - Non-type template parameters of type 'std::nullptr_t' were not
  permitted.
  - We didn't properly introduce built-in operators for nullptr ==,
  !=, <, <=, >=, or > as candidate functions .

To my knowledge, there's only one (minor but annoying) part of nullptr
that hasn't been implemented: catching a thrown 'nullptr' as a pointer
or pointer-to-member, per C++0x [except.handle]p4.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131813 91177308-0d34-0410-b5e6-96231b3b80d8