swift234

Teach SILGen to generate debug_value and debug_value_addr for 'let' decls.

Right now I unconditionally generate these, because silgen doesn't have
easy access to whether "-g" is passed or not.  We can either make SILGen
know about this, or we can have optimizations strip these uses out.
Opinions welcome!

As an example, something like:
protocol P { func bonk() }

func foo(let a : Int, let b : P) -> Int {
  let x = a
  b.bonk()
  return x
}

compiles into:

sil @_TF1t3fooFT1aSi1bPS_1P__Si : $@thin (Int64, @in P) -> Int64 {
bb0(%0 : $Int64, %1 : $*P):
  debug_value %0 : $Int64  // let a               // id: %2
  debug_value_addr %1 : $*P  // let b             // id: %3
  debug_value %0 : $Int64  // let x               // id: %4
  %5 = project_existential %1 : $*P to $*@sil_self P // user: %7
  %6 = protocol_method %1 : $*P, #P.bonk!1 : $@cc(witness_method) @callee_owned (@inout Self) -> () // user: %7
  %7 = apply %6(%5) : $@cc(witness_method) @callee_owned (@inout Self) -> ()
  destroy_addr %1 : $*P                           // id: %8
  return %0 : $Int64                              // id: %9
}



Swift SVN r12193