[NFC] Grab the private discriminator for a file in IRGenDebugInfo

This was being done at an odd point in the frontend presumably because by that point the private discriminator had been fully computed.  Instead, push the conditions for generating the prefix data down to debug info generation and stop mutating IRGenOptions::DebugFlag in the frontend.

include/swift/Subsystems.h

@@ -284,6 +284,7 @@ namespace swift {
   performIRGeneration(IRGenOptions &Opts, SourceFile &SF,
                       std::unique_ptr<SILModule> SILMod,
                       StringRef ModuleName, const PrimarySpecificPaths &PSPs,
+                      StringRef PrivateDiscriminator,
                       llvm::LLVMContext &LLVMContext,
                       llvm::GlobalVariable **outModuleHash = nullptr,
                       llvm::StringSet<> *LinkerDirectives = nullptr);