-Rename -Wargument-larger-than -> -Wlarge-by-value-copy

-Improve the diagnostic message
-Add some comments

Suggestions by Chris.

llvm-svn: 119594

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1373,7 +1373,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.MathErrno = Args.hasArg(OPT_fmath_errno);
   Opts.InstantiationDepth = Args.getLastArgIntValue(OPT_ftemplate_depth, 1024,
                                                Diags);
-  Opts.ArgumentLargerThan = Args.getLastArgIntValue(OPT_Wargument_larger_than,
+  Opts.NumLargeByValueCopy = Args.getLastArgIntValue(OPT_Wlarge_by_value_copy,
                                                     0, Diags);
   Opts.NeXTRuntime = !Args.hasArg(OPT_fgnu_runtime);
   Opts.ObjCConstantStringClass =