Trigger SIL parsing from performSILGeneration

Rather than eagerly parsing an input .sil file
in `performSemaUpTo`, trigger it from
`performSILGeneration`. This will allow us to
remove the SILModule stored on the
CompilerInstance and will eventually allow the
various SIL tools to just call into
`performSILGeneration` without needing to call
`performSema`.

lib/Frontend/Frontend.cpp

@@ -814,13 +814,6 @@ void CompilerInstance::performSemaUpTo(SourceFile::ASTStage_t LimitStage) {
 
   forEachFileToTypeCheck([&](SourceFile &SF) {
     performTypeChecking(SF);
-
-    // Parse the SIL decls if needed.
-    // TODO: Requestify SIL parsing.
-    if (TheSILModule) {
-      SILParserState SILContext(TheSILModule.get());
-      parseSourceFileSIL(SF, &SILContext);
-    }
   });
 
   finishTypeChecking();