add the overlay module name to the overlay symbol graph file name

Author: QuietMisdreavus

lib/SymbolGraphGen/SymbolGraphGen.cpp

@@ -24,12 +24,18 @@ int serializeSymbolGraph(SymbolGraph &SG,
                          const SymbolGraphOptions &Options) {
   SmallString<256> FileName;
   if (SG.DeclaringModule.hasValue()) {
-    // Save a cross-import overlay symbol graph as `MainModule@BystandingModule[@BystandingModule...].symbols.json`
+    // Save a cross-import overlay symbol graph as `MainModule@BystandingModule[@BystandingModule...]@OverlayModule.symbols.json`
+    //
+    // The overlay module's name is added as a disambiguator in case an overlay
+    // declares multiple modules for the same set of imports.
     FileName.append(SG.DeclaringModule.getValue()->getNameStr());
     for (auto BystanderModule : SG.BystanderModules) {
       FileName.push_back('@');
       FileName.append(BystanderModule.str());
     }
+    
+    FileName.push_back('@');
+    FileName.append(SG.M.getNameStr());
   } else {
     FileName.append(SG.M.getNameStr());
 

test/SymbolGraph/Module/CrossImport.swift

@@ -4,7 +4,7 @@
 // RUN: %target-build-swift %s -module-name _A_B -I %t -emit-module -emit-module-path %t/
 // RUN: cp -r %S/Inputs/CrossImport/A.swiftcrossimport %t/
 // RUN: %target-swift-symbolgraph-extract -module-name A -I %t -pretty-print -output-dir %t
-// RUN: %FileCheck %s --input-file %t/[email protected]
+// RUN: %FileCheck %s --input-file %t/A@B@_A_B.symbols.json
 
 @_exported import A
 import B