[SIL] Remove TranslationUnit pointer from SILModule.

This reverts r8624 and compensates by passing the TU to the SILModule printer when needed.

This addresses concerns that Jordan and Sean had raised.

Swift SVN r8678

lib/SIL/SILModule.cpp

@@ -38,10 +38,8 @@ namespace swift {
 /// SILModule that these things are uniqued into.
 typedef llvm::FoldingSet<SILTypeList> SILTypeListUniquingType;
 
-SILModule::SILModule(TranslationUnit *TU)
-  : ASTTranslUnit(TU),
-    Stage(SILStage::Raw),
-    Types(*this)
+SILModule::SILModule(ASTContext &Context)
+  : TheASTContext(Context), Stage(SILStage::Raw), Types(*this)
 {
   TypeListUniquing = new SILTypeListUniquingType();
 }