Implement support for comparing pointers with <, >, <=, >=, ==, and !=

Implement support for comparing pointers with <, >, <=, >=, ==, and !=
in C++, taking into account conversions to the "composite pointer
type" so that we can compare, e.g., a pointer to a derived class to a
pointer to a base class.

Also, upgrade the "comparing distinct pointer types" from a warning to
an error for C++, since this is clearly an error. Turns out that we
hadn't gone through and audited this code for C++, ever.

Fixes <rdar://problem/6816420>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70829 91177308-0d34-0410-b5e6-96231b3b80d8