Initial, rudimentary implementation of operator overloading for binary

operators. For example, one can now write "x + y" where x or y is a
class or enumeration type, and Clang will perform overload resolution
for "+" based on the overloaded operators it finds.

The other kinds of overloadable operators in C++ will follow this same
approach. 

Three major issues remain:
  1) We don't find member operators
  2) Since we don't have user-defined conversion operators, we can't
  call any of the built-in overloaded operators in C++ [over.built].
  3) Once we've done the semantic checks, we drop the overloaded
  operator on the floor; it doesn't get into the AST at all.

llvm-svn: 58821