Added operator overloading for unary operators, post-increment, and

post-decrement, including support for generating all of the built-in
operator candidates for these operators. 

C++ and C have different rules for the arguments to the builtin unary
'+' and '-'. Implemented both variants in Sema::ActOnUnaryOp.

In C++, pre-increment and pre-decrement return lvalues. Update
Expr::isLvalue accordingly.

llvm-svn: 59638