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More typechecking, refactoring...
- Implemented the following routines...CheckAdditiveOperands,
CheckCommaOperands, CheckLogicalOperands.
- Added maxComplexType, maxFloatingType, & maxIntegerType to ASTContext.
Ranking helper functions moved to ASTContext as well (they are private:-)
- Simplified UsualArithmeticConversions using the new ASTContext hooks.
- Fixed isAssignmentOp()...is was preventing comma exprs from ever being created:-(
- Changed a GCC compat extension to truly be an extension (and turned extensions
on by default). This now produces a warning by default.

llvm-svn: 39418