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Implement ++/-- typechecking for pre/post unary expressions. This includes:

- added isLvalue, isModifiableLvalue (on TypeRef, Type, and RecordType).
- added isRealType, isRealFloatingType, isComplexType.
- hacked Diag to take a TypeRef (I was sick of writing the 2 line "setup":-)
In addition, this will likely lead to less bugs...I already had written code
that was doing a getAsString on "Type" (which is wrong...since it doesn't include
any qualifiers).
- Changed UnaryOperator to take a TypeRef...pass it the right stuff.
- Removed redundant ternary expressions in several predicates.
- A couple diagnostics.

llvm-svn: 39369