Implement LLVM IR Witness Method Elimination for Swift witness tables. (#39287)

- Witness method calls are done via @llvm.type.checked.load instrinsic call with a type identifier
- Type id of a witness method is the requirement's mangled name
- Witness tables get !type markers that list offsets and type ids of all methods in the wtable
- Added -enable-llvm-wme to enable Witness Method Elimination
- Added IR test and execution test

lib/Frontend/CompilerInvocation.cpp

@@ -1908,6 +1908,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.VirtualFunctionElimination = true;
   }
 
+  if (Args.hasArg(OPT_enable_llvm_wme)) {
+    Opts.WitnessMethodElimination = true;
+  }
+
   // Default to disabling swift async extended frame info on anything but
   // darwin. Other platforms are unlikely to have support for extended frame
   // pointer information.