[IRGen] Fix SynthesizedFileUnit. (#31082)

SynthesizedFileUnits are attached to a SourceFile.

Make IRGen consistent with TBDGen: when processing a SourceFile, only process
the attached SynthesizedFile. This avoids IRGen/TBDGen inconsistencies.

Use SourceFile name in SynthesizedFileUnit::getDiscriminatorForPrivateValue.

Resolves TF-1249.

Author: dan-zheng

lib/AST/Module.cpp

@@ -2694,16 +2694,21 @@ SynthesizedFileUnit::getDiscriminatorForPrivateValue(const ValueDecl *D) const {
   if (!PrivateDiscriminator.empty())
     return PrivateDiscriminator;
 
-  assert(1 == count_if(getParentModule()->getFiles(),
-                       [](const FileUnit *FU) -> bool {
-                         return isa<SynthesizedFileUnit>(FU);
-                       }) &&
-         "Cannot promise uniqueness if multiple synthesized file units exist");
+  StringRef sourceFileName = getSourceFile().getFilename();
+  if (sourceFileName.empty()) {
+    assert(1 == count_if(getParentModule()->getFiles(),
+                         [](const FileUnit *FU) -> bool {
+                           return isa<SourceFile>(FU) &&
+                                  cast<SourceFile>(FU)->getFilename().empty();
+                         }) &&
+           "Cannot promise uniqueness if multiple source files are nameless");
+  }
 
   // Use a discriminator invariant across Swift version: a hash of the module
-  // name and a special string.
+  // name, the parent source file name, and a special string.
   llvm::MD5 hash;
   hash.update(getParentModule()->getName().str());
+  hash.update(llvm::sys::path::filename(sourceFileName));
   // TODO: Use a more robust discriminator for synthesized files. Pick something
   // that cannot conflict with `SourceFile` discriminators.
   hash.update("SYNTHESIZED FILE");

lib/IRGen/IRGen.cpp

@@ -982,11 +982,16 @@ performIRGeneration(const IRGenOptions &Opts, ModuleDecl *M,
 
     if (SF) {
       IGM.emitSourceFile(*SF);
+      // Emit synthesized file unit, if it exists.
+      if (auto *synthesizedFile = SF->getSynthesizedFile())
+        IGM.emitSynthesizedFileUnit(*synthesizedFile);
     } else {
       for (auto *File : M->getFiles()) {
         if (auto *nextSF = dyn_cast<SourceFile>(File)) {
           if (nextSF->ASTStage >= SourceFile::TypeChecked)
             IGM.emitSourceFile(*nextSF);
+        } else if (auto *nextSFU = dyn_cast<SynthesizedFileUnit>(File)) {
+          IGM.emitSynthesizedFileUnit(*nextSFU);
         } else {
           File->collectLinkLibraries([&IGM](LinkLibrary LinkLib) {
             IGM.addLinkLibrary(LinkLib);
@@ -995,11 +1000,6 @@ performIRGeneration(const IRGenOptions &Opts, ModuleDecl *M,
       }
     }
 
-    // Emit synthesized file units.
-    for (auto *File : M->getFiles())
-      if (auto *nextSFU = dyn_cast<SynthesizedFileUnit>(File))
-        IGM.emitSynthesizedFileUnit(*nextSFU);
-
     // Okay, emit any definitions that we suddenly need.
     irgen.emitLazyDefinitions();