[Caching] Allow prefix mapping for generated bridging header

To allow prefix mapping of the bridging header to achieve cache hit when
source files are located in different location, the generated chained
bridging header should not include absolute paths of the headers. Fix
the problem by concat the chained bridging header together.

Fixes: https://github.com/swiftlang/swift/issues/84088

lib/Serialization/ModuleFileSharedCore.h

@@ -675,9 +675,9 @@ public:
   }
 
   /// Get embedded bridging header.
-  std::string getEmbeddedHeader() const {
+  StringRef getEmbeddedHeader() const {
     // Don't include the '\0' in the end.
-    return importedHeaderInfo.contents.drop_back().str();
+    return importedHeaderInfo.contents.drop_back();
   }
 
   /// If the module-defining `.swiftinterface` file is an SDK-relative path,