[Options] Add prefixes to options.

Each option has a set of prefixes. When matching an argument such as
-funroll-loops. First the leading - is removed as it is a prefix. Then
a lower_bound search for "funroll-loops" is done against the option table by
option name. From there each option prefix + option name combination is tested
against the argument.

This allows us to support Microsoft style options where both / and - are valid
prefixes. It also simplifies the cases we already have where options come in
both - and -- forms. Almost every option for gnu-ld happens to have this form.

llvm-svn: 166444

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1001,10 +1001,10 @@ static void addWarningArgs(ArgList &Args, std::vector<std::string> &Warnings) {
   for (arg_iterator I = Args.filtered_begin(OPT_W_Group),
          E = Args.filtered_end(); I != E; ++I) {
     Arg *A = *I;
-    // If the argument is a pure flag, add its name (minus the "-W" at the beginning)
+    // If the argument is a pure flag, add its name (minus the "W" at the beginning)
     // to the warning list. Else, add its value (for the OPT_W case).
     if (A->getOption().getKind() == Option::FlagClass) {
-      Warnings.push_back(A->getOption().getName().substr(2));
+      Warnings.push_back(A->getOption().getName().substr(1));
     } else {
       for (unsigned Idx = 0, End = A->getNumValues();
            Idx < End; ++Idx) {