Implement LLVM IR Virtual Function Elimination for Swift classes. (#39128)

- Virtual calls are done via a @llvm.type.checked.load instrinsic call with a type identifier
- Type identifier of a vfunc is the base method's mangling
- Type descriptors and class metadata get !type markers that list offsets and type identifiers of all vfuncs
- The -enable-llvm-vfe frontend flag enables VFE
- Two added tests verify the behavior on IR and by executing a program

lib/Frontend/CompilerInvocation.cpp

@@ -1896,6 +1896,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.EnableGlobalISel = true;
   }
 
+  if (Args.hasArg(OPT_enable_llvm_vfe)) {
+    Opts.VirtualFunctionElimination = true;
+  }
+
   return false;
 }