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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59638 91177308-0d34-0410-b5e6-96231b3b80d8