swift234

SIL: Key functions directly without SILConstant.

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