Sema: Centralize check for -experimental-lazy-typecheck.

Always check for the flag before evaluating `TypeCheckSourceFileRequest`,
rather than just in `performSema()`. This ensures later compilation stages that
re-request eager typechecking honor the flag.

Author: tshortli

lib/Frontend/Frontend.cpp

@@ -1425,11 +1425,6 @@ bool CompilerInstance::performParseAndResolveImportsOnly() {
 void CompilerInstance::performSema() {
   performParseAndResolveImportsOnly();
 
-  // Skip eager type checking. Instead, let later stages of compilation drive
-  // type checking as needed through request evaluation.
-  if (getASTContext().TypeCheckerOpts.EnableLazyTypecheck)
-    return;
-
   FrontendStatsTracer tracer(getStatsReporter(), "perform-sema");
 
   forEachFileToTypeCheck([&](SourceFile &SF) {

lib/Sema/TypeChecker.cpp

@@ -242,6 +242,12 @@ void swift::bindExtensions(ModuleDecl &mod) {
 }
 
 void swift::performTypeChecking(SourceFile &SF) {
+  if (SF.getASTContext().TypeCheckerOpts.EnableLazyTypecheck) {
+    // Skip eager type checking. Instead, let later stages of compilation drive
+    // type checking as needed through request evaluation.
+    return;
+  }
+
   return (void)evaluateOrDefault(SF.getASTContext().evaluator,
                                  TypeCheckSourceFileRequest{&SF}, {});
 }