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Implemented type checking for compound assignments (*=, /=, etc.).

This encouraged me to do a fairly dramatic refactoring of the Check* functions.
(since I wanted to reuse the existing work, rather than duplicate the logic).

For example, I changed all the Check* functions to return a QualType (instead
of returning an Expr). This had a very nice side benefit...there is now
only one instantiation point for BinaryOperator()! (A property I've always
wanted...separating type checking from AST building is *much* nicer). Another
change is to remove "code" from all the Check* functions (this allowed
me to remove the weird comment about enums/unsigned:-). Removing the
code forced me to add a few functions, however. For example,

<   ExprResult CheckAdditiveOperands( // C99 6.5.6
<     Expr *lex, Expr *rex, SourceLocation OpLoc, unsigned OpCode);

>   inline QualType CheckAdditionOperands( // C99 6.5.6
>     Expr *lex, Expr *rex, SourceLocation OpLoc);
>   inline QualType CheckSubtractionOperands( // C99 6.5.6
>     Expr *lex, Expr *rex, SourceLocation OpLoc);

While this isn't as terse, it more closely reflects the differences in
the typechecking logic. For example, I disliked having to check the code again
in CheckMultiplicativeOperands/CheckAdditiveOperands.

Created the following helper functions:
- Expr::isNullPointerConstant().
- SemaExpr.cpp: static inline BinaryOperator::Opcode ConvertTokenKindToBinaryOpcode().
This was purely asethetic, since ParseBinOp() is now larger. I didn't feel
like looking at 2 huge switch statements. ParseBinOp() now avoids using
any of the BinaryOperator predicates (since I switched to a switch statement:-)

Only one regret (minor). I couldn't figure out how to avoid having two assign functions,
CheckCompoundAssignmentOperands, CheckSimpleAssignmentOperands. Conceptually,
the two functions make sense. Unfortunately, their implementation contains a lot of
duplication (thought they aren't that be in the first place).

llvm-svn: 39433