Virtualize swift compiler outputs (#63206)

Using a virutal output backend to capture all the outputs from
swift-frontend invocation. This allows redirecting and/or mirroring
compiler outputs to multiple location using different OutputBackend.

As an example usage for the virtual outputs, teach swift compiler to
check its output determinism by running the compiler invocation
twice and compare the hash of all its outputs.

Virtual output will be used to enable caching in the future.

lib/Frontend/CompilerInvocation.cpp

@@ -2364,6 +2364,7 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   Opts.UseProfile = ProfileUse ? ProfileUse->getValue() : "";
 
   Opts.PrintInlineTree |= Args.hasArg(OPT_print_llvm_inline_tree);
+  Opts.AlwaysCompile |= Args.hasArg(OPT_always_compile_output_files);
 
   Opts.EnableDynamicReplacementChaining |=
       Args.hasArg(OPT_enable_dynamic_replacement_chaining);