This frees up an extra bit in SILType, which we can expose to LLVM through
PointerLikeTypeTraits. Use this bit in a PointerUnion, which allows simplifying
ValueBase, which happened to be the last use of the isInvalid() state.
Swift SVN r5218
Replace 'constant_ref' with 'function_ref', which references a SILFunction directly, and 'global_addr', which references a global variable VarDecl. Get rid of the SILConstant-to-SILFunction mapping in SILModule and replace it with an ilist of SILFunctions. Allow SILFunctions to be 'external' by not having any blocks in their body.
For now, SILFunctions still carry around their SILConstant "name", because name mangling and IRGen still rely on access to the original decl in order to recover IRGen information, which unfortunately leaves IRGen's CodeRefs in a gross, awkward intermediate state. Lifting mangling, AbstractCC, and other linkage attributes to SIL should clear up this up.
Swift SVN r4865
When lowering SIL builtin ConstantRefs, just stash away the FuncDecl, and pass that decl on to a tweaked version of emitBuiltinCall when the constant is applied.
Swift SVN r4736
take a const ValueBase* instead of a SILValue, implement SILArgument
cases for a few visitors and opt others out explicitly, and assert
that classes in the SIL value hierarchy override their superclass's
classof.
Swift SVN r4705
