Serialize symbol graphs for extended modules separately

When a module extends a type from another module, serialize those symbols into
separated files dedicated to those extended modules. This makes it easier to
ingest and categorize those symbols under the extended module if desired.

rdar://58941718

include/swift/SymbolGraphGen/SymbolGraphGen.h

@@ -20,8 +20,8 @@ class ModuleDecl;
 namespace symbolgraphgen {
 
 struct SymbolGraphOptions {
-  /// The path to output the symbol graph JSON.
-  StringRef OutputPath;
+  /// The directory to output the symbol graph JSON files.
+  StringRef OutputDir;
 
   /// The target of the module.
   llvm::Triple Target;