Implement enums for Cmm

Using ANSI C enums is documented as a preferred way of defining constants: https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Conventions#Syntacticdetails

Basically instead of writing:

#define INVALID_OBJECT 0
#define CONSTR 1

you would write:

typedef enum {
 INVALID_OBJECT = 0,
 CONSTR = 1
} ClosureType;

What you get is easier debugging in gdb, perhaps even better type checking.

Unfortunately some files (most vexingly rts/storage/ClosureTypes.h) are included in both C and Cmm and Cmm doesn't support this syntax. As far as I can tell all constants in Cmm are defined with preprocessor #defines.

Implementing enums/constants may seem like an overkill, but I haven't found a good workaround for this.

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Trac field	Value
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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (CodeGen)
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