Split IRGen's value-witness emission code into its own file. NFC.

Author: rjmccall

lib/IRGen/CMakeLists.txt

@@ -25,6 +25,7 @@ add_swift_library(swiftIRGen
   GenStruct.cpp
   GenTuple.cpp
   GenType.cpp
+  GenValueWitness.cpp
   IRGen.cpp
   IRGenDebugInfo.cpp
   IRGenFunction.cpp

lib/IRGen/GenMeta.cpp

@@ -43,6 +43,7 @@
 #include "FixedTypeInfo.h"
 #include "GenClass.h"
 #include "GenPoly.h"
+#include "GenValueWitness.h"
 #include "GenArchetype.h"
 #include "GenStruct.h"
 #include "HeapTypeInfo.h"