[SILGen] Allow lazy type-checking

Remove the type-checking call from
OptimizedIRRequest, and sink it down into SILGen
and IRGen when we come to emit the source files.

lib/IRGen/GenDecl.cpp

@@ -34,6 +34,7 @@
 #include "swift/SIL/FormalLinkage.h"
 #include "swift/SIL/SILDebugScope.h"
 #include "swift/SIL/SILModule.h"
+#include "swift/Subsystems.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/GlobalDecl.h"
@@ -443,7 +444,10 @@ class PrettySynthesizedFileUnitEmission : public llvm::PrettyStackTraceEntry {
 
 /// Emit all the top-level code in the source file.
 void IRGenModule::emitSourceFile(SourceFile &SF) {
-  assert(SF.ASTStage == SourceFile::TypeChecked);
+  // Type-check the file if we haven't already (this may be necessary for .sil
+  // files, which don't get fully type-checked by parsing).
+  performTypeChecking(SF);
+
   PrettySourceFileEmission StackEntry(SF);
 
   // Emit types and other global decls.

lib/IRGen/IRGen.cpp

@@ -1447,12 +1447,6 @@ GeneratedModule OptimizedIRRequest::evaluate(Evaluator &evaluator,
 
   bindExtensions(*parentMod);
 
-  // Type-check the files that need emitting.
-  for (auto *file : desc.getFiles()) {
-    if (auto *SF = dyn_cast<SourceFile>(file))
-      performTypeChecking(*SF);
-  }
-
   if (ctx.hadError())
     return GeneratedModule::null();
 

lib/SILGen/SILGen.cpp

@@ -1884,7 +1884,8 @@ class SILGenModuleRAII {
 
 public:
   void emitSourceFile(SourceFile *sf) {
-    assert(sf->ASTStage == SourceFile::TypeChecked);
+    // Type-check the file if we haven't already.
+    performTypeChecking(*sf);
 
     SourceFileScope scope(SGM, sf);
     for (Decl *D : sf->getTopLevelDecls()) {